Please refer to the attached sample project. The ResizeColumnsProportionally.gif illustrates the correct behavior of columns resizing. However, if you make a certain column with fixed size, the columns resizing is not proportional any more. The ColumnsWidthNOTAdjustedProportionally.gif demonstrates better the wrong behavior.

Workaround:  handle the SizeChanged event and adjust the columns' width property programmatically in order to obtain the desired widths.

To reproduce:
Public Class RadForm1
    Public RadGridView1 As RadGridView

    Private Sub RadForm1_Load(sender As Object, e As EventArgs) Handles Me.Load
        CreateGrid()
        FormatGrid()
        SetDataSource()
    End Sub

    Private Sub CreateGrid()
        Try
            RadGridView1 = New RadGridView
            Me.Controls.Add(RadGridView1)
        Catch ex As Exception
            MessageBox.Show(Me, ex.Message)
        End Try
    End Sub
    Private Sub FormatGrid()

        Dim col As GridViewTextBoxColumn
        With RadGridView1
            col = New GridViewTextBoxColumn
            With col
                .FieldName = "FieldID"
                'NOTE: Comment out NullValue = "" to prevent the unhandled exception
                .NullValue = ""
            End With
            .Columns.Add(col)
        End With
    End Sub
    Private Sub SetDataSource()
        Dim table As DataTable = Nothing
        Dim row As DataRow = Nothing
        table = New DataTable("GridList")
        table.Columns.Add("FieldID", GetType(System.Guid))
        row = table.NewRow
        row("FieldID") = Guid.Empty
        table.Rows.Add(row)
        RadGridView1.DataSource = table
    End Sub
End Class

Workaround:
Comment this line 
NullValue = ""

Use attached to reproduce.

Workaround
- Set the position at runtime. 
   Me.radGridView1.MasterTemplate.AddNewRowPosition = Telerik.WinControls.UI.SystemRowPosition.Bottom

To reproduce: if you pin a calculator or a hyperlink column you will notice that the cells belonging to these columns remain white. However, the cells from other column types  have a light blue/gray fill color for pinned state. 

Workaround: 
private void RadGridView1_CellFormatting(object sender, CellFormattingEventArgs e)
{
    if (e.Column.IsPinned)
    {
        e.CellElement.BackColor = Color.FromArgb(235, 244, 252);
    }
    else
    {
        e.CellElement.ResetValue(LightVisualElement.BackColorProperty, ValueResetFlags.Local);
    }
}

To reproduce: please refer to the attached screenshot.

Workaround: add the FilterDescriptor programmatically: https://docs.telerik.com/devtools/winforms/gridview/filtering/setting-filters-programmatically-(simple-descriptors)

Excel does not support dates prior to 1/1/1900, however,  we could export such dates as simple strings: https://support.office.com/en-us/article/excel-specifications-and-limits-1672b34d-7043-467e-8e27-269d656771c3

How to reproduce:
 public partial class Form1 : RadForm
 {
     public Form1()
     {
         InitializeComponent();

         this.radGridView1.DataSource = this.GetData();
         this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;
     }

     private object GetData()
     {
         DataTable dt = new DataTable();
         dt.Columns.Add("Id", typeof(int));
         dt.Columns.Add("Name", typeof(string));
         dt.Columns.Add("Bool", typeof(bool));
         dt.Columns.Add("Date", typeof(DateTime));

         DateTime date = new DateTime(1850, 1, 1);
         for (int i = 0; i < 100; i++)
         {
             dt.Rows.Add(i, "Name " + i, i % 2 == 0, date.AddYears(i));
         }

         return dt;
     }
     
     FieldInfo fi;
     private void radButton1_Click(object sender, EventArgs e)
     {
         //Old Export using the ExcelML format
         this.fi = typeof(Telerik.WinControls.UI.Export.ExcelML.CellElement).GetField("_dataElement", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic);
         ExportToExcelML exporter = new ExportToExcelML(this.radGridView1);
         string fileName = @"..\..\data.xls";
         exporter.RunExport(fileName);

         //New export utilizing the SpreadProcessing libraries
         GridViewSpreadExport spreadExporter = new GridViewSpreadExport(this.radGridView1);

         SpreadExportRenderer exportRenderer = new SpreadExportRenderer();
         spreadExporter.RunExport(@"..\..\data.xlsx", exportRenderer);
     }
 }


Workaround: handle the CellFormatting event
public partial class Form1 : RadForm
{
    public Form1()
    {
        InitializeComponent();

        this.radGridView1.DataSource = this.GetData();
        this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;
    }

    private object GetData()
    {
        DataTable dt = new DataTable();
        dt.Columns.Add("Id", typeof(int));
        dt.Columns.Add("Name", typeof(string));
        dt.Columns.Add("Bool", typeof(bool));
        dt.Columns.Add("Date", typeof(DateTime));

        DateTime date = new DateTime(1850, 1, 1);
        for (int i = 0; i < 100; i++)
        {
            dt.Rows.Add(i, "Name " + i, i % 2 == 0, date.AddYears(i));
        }

        return dt;
    }
    
    FieldInfo fi;
    private void radButton1_Click(object sender, EventArgs e)
    {
        //Old Export using the ExcelML format
        this.fi = typeof(Telerik.WinControls.UI.Export.ExcelML.CellElement).GetField("_dataElement", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic);
        ExportToExcelML exporter = new ExportToExcelML(this.radGridView1);
        exporter.ExcelCellFormatting += Exporter_ExcelCellFormatting;
        string fileName = @"..\..\data.xls";
        exporter.RunExport(fileName);

        //New export utilizing the SpreadProcessing libraries
        GridViewSpreadExport spreadExporter = new GridViewSpreadExport(this.radGridView1);
         spreadExporter.CellFormatting += SpreadExporter_CellFormatting;

        SpreadExportRenderer exportRenderer = new SpreadExportRenderer();
        spreadExporter.RunExport(@"..\..\data.xlsx", exportRenderer);
    }

    private void SpreadExporter_CellFormatting(object sender, Telerik.WinControls.Export.CellFormattingEventArgs e)
    {
        if (e.GridRowIndex >= 1 && e.GridCellInfo.ColumnInfo is GridViewDateTimeColumn && ((DateTime)e.GridCellInfo.Value).Year < 1900)
        {
            Telerik.Windows.Documents.Spreadsheet.Model.CellSelection cell = e.CellSelection as Telerik.Windows.Documents.Spreadsheet.Model.CellSelection;
            cell.SetValue(e.GridCellInfo.Value.ToString());
        }
    }

    private void Exporter_ExcelCellFormatting(object sender, Telerik.WinControls.UI.Export.ExcelML.ExcelCellFormattingEventArgs e)
    {
        if (e.GridRowIndex > -1 && e.GridCellInfo.ColumnInfo is GridViewDateTimeColumn && ((DateTime)e.GridCellInfo.Value).Year < 1900)
        {
            DataElement data = new DataElement();
            data.DataItem = e.GridCellInfo.Value.ToString();
            this.fi.SetValue(e.ExcelCellElement, data);
        }
    }
}

Bind the grid to the following DataTable:

            DataTable dt = new DataTable();     
            dt.Columns.Add("X.Y"); 
            dt.Columns.Add("X");   
            
            dt.Rows.Add("Data X.Y", "Data X");

            this.radGridView1.DataSource = dt;

You will notice that the second column "X" is missing.

Note: if you change the order of adding the columns, both columns will be added.

To reproduce:
 this.radGridView1.SummaryRowsBottom.Insert(0, summaryRowItem2);
 this.radGridView1.SummaryRowsBottom.Move(2, 0);

Workaround:
Remove all items and add them back with a spesific order. 

How to reproduce: check the attached project and video

Please refer to the attached sample project and follow the steps illustrated in the gif file.

Note: the same behavior is observed with the CellValidating event in the new row.

Workaround: use the RowValidating event instead:

        private void radGridView1_RowValidating(object sender, Telerik.WinControls.UI.RowValidatingEventArgs e)
        {
            if (e.Row is Telerik.WinControls.UI.GridViewNewRowInfo && e.Row.Cells["Id"].Value == null)
            {
                this.radGridView1.MasterView.TableAddNewRow.CancelAddNewRow();
                this.radGridView1.CurrentRow = this.radGridView1.MasterView.TableAddNewRow;
            }
        }
 
NOTE: The CellValidating event is fired multiple times as well when you select a new tab in RadDock. There is a second project attached.

To reproduce:

            this.radGridView1.Columns.Add("FIRST CELL ID A");

            this.radGridView1.Rows.Add("405-55-214-41763");
            this.radGridView1.Rows.Add("405-55-214-46682");
            this.radGridView1.Rows.Add("405-55-214-46682");

            this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;
            this.radGridView1.MultiSelect = true;
            this.radGridView1.SelectionMode = Telerik.WinControls.UI.GridViewSelectionMode.CellSelect;

Please refer to the attached gif file illustrating better the text moving. 

Workaround: 
        private void radGridView1_CellFormatting(object sender, Telerik.WinControls.UI.CellFormattingEventArgs e)
        {
            e.CellElement.BorderBottomWidth = 2;
            e.CellElement.BorderRightWidth = 2;
            e.CellElement.BorderTopWidth = 2;
            e.CellElement.BorderLeftWidth = 2;
        }

The new mode should allow copying of single cells even when having the SelectionMode set to FullRowSelect. The attached project features a possible workaround.

To reproduce: run the attached project and toggle the checkbox in the header cell of the child template. 

Workaround: use a custom GridCheckBoxHeaderCellElement


    public partial class RadForm1 : Telerik.WinControls.UI.RadForm
    {
        public RadForm1()
        {
            InitializeComponent();

            this.radGridView1.CreateCell += radGridView1_CreateCell;

            DataTable dt = new DataTable();
            dt.Columns.Add("CategoryId", typeof(int));
            dt.Columns.Add("CategoryName", typeof(string));
            dt.Columns.Add("ParentCategoryId", typeof(int));

            dt.Rows.Add(1, "Category1", 0);
            dt.Rows.Add(2, "Category2", 0);
            dt.Rows.Add(3, "Category3", 0);

            Random rand = new Random();
            for (int i = 4; i < 20; i++)
            {
                dt.Rows.Add(i, "Category" + i, rand.Next(1, 4));
            }
            this.radGridView1.Relations.AddSelfReference(this.radGridView1.MasterTemplate, "CategoryId", "ParentCategoryId");
            this.radGridView1.DataSource = dt;
            this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;

            GridViewTemplate childTemplate = CreateChildTemplate();
            GridViewRelation relation = new GridViewRelation(
                this.radGridView1.MasterTemplate,
                childTemplate);
            relation.ChildColumnNames.Add("CategoryId");
            relation.ParentColumnNames.Add("CategoryId");
            this.radGridView1.Relations.Add(relation);
        }

        private void radGridView1_CreateCell(object sender, GridViewCreateCellEventArgs e)
        {
            if (e.CellType == typeof(GridCheckBoxHeaderCellElement))
            {
                e.CellElement = new CustomGridCheckBoxHeaderCellElement(e.Column, e.Row);
            }
        }

        private GridViewTemplate CreateChildTemplate()
        {
            GridViewTemplate childTemplate = new GridViewTemplate();
            childTemplate.AutoGenerateColumns = false;
            this.radGridView1.Templates.Add(childTemplate);

            GridViewDecimalColumn decColumn = new GridViewDecimalColumn
            {
                Name = "CategoryId",
                HeaderText = "CategoryId Id",
                FieldName = "CategoryId",
                IsVisible = false,
                MinWidth = 100
            };
            childTemplate.Columns.Add(decColumn);

            GridViewTextBoxColumn tbxColumn = new GridViewTextBoxColumn
            {
                Name = "RightName",
                HeaderText = "Right Name",
                FieldName = "RGT_NAME",
                ReadOnly = true,
                MinWidth = 100
            };
            childTemplate.Columns.Add(tbxColumn);

            GridViewCheckBoxColumn chkxColumn = new GridViewCheckBoxColumn
            {
                Name = "Checkbox",
                EnableHeaderCheckBox = true,
                FieldName = "HasAccess",
            };
            childTemplate.Columns.Add(chkxColumn);
            childTemplate.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
            return childTemplate;
        }

        public class CustomGridCheckBoxHeaderCellElement : GridCheckBoxHeaderCellElement
        {
            public CustomGridCheckBoxHeaderCellElement(GridViewColumn column, GridRowElement row) : base(column, row)
            {
            }

            public bool SuspendProcessingToggleStateChanged
            {
                get
                {
                    FieldInfo fi = typeof(GridCheckBoxHeaderCellElement).GetField("suspendProcessingToggleStateChanged", BindingFlags.NonPublic | BindingFlags.Instance);
                    return (bool)fi.GetValue(this);
                }
                set
                {
                    FieldInfo fi = typeof(GridCheckBoxHeaderCellElement).GetField("suspendProcessingToggleStateChanged", BindingFlags.NonPublic | BindingFlags.Instance);
                    fi.SetValue(this, value);
                }
            }

            public bool ShouldCheckDataRows
            {
                get
                {
                    FieldInfo fi = typeof(GridCheckBoxHeaderCellElement).GetField("shouldCheckDataRows", BindingFlags.NonPublic | BindingFlags.Instance);
                    return (bool)fi.GetValue(this);
                }
                set
                {
                    FieldInfo fi = typeof(GridCheckBoxHeaderCellElement).GetField("shouldCheckDataRows", BindingFlags.NonPublic | BindingFlags.Instance);
                    fi.SetValue(this, value);
                }
            }

            protected override void checkbox_ToggleStateChanged(object sender, StateChangedEventArgs args)
            {
                if (SuspendProcessingToggleStateChanged)
                {
                    return;
                }

                if (this.ViewTemplate != null && !this.ViewTemplate.IsSelfReference && !this.MasterTemplate.IsSelfReference)
                {
                    this.MasterTemplate.BeginUpdate();
                }
                else
                {
                    this.TableElement.BeginUpdate();
                }

                object valueState = DBNull.Value;

                if (args.ToggleState == ToggleState.On)
                {
                    valueState = true;
                }
                else if (args.ToggleState == ToggleState.Off)
                {
                    valueState = false;
                }
                else if (args.ToggleState == ToggleState.Indeterminate)
                {
                    valueState = null;
                }

                RaiseToggleStateEvent();

                if (!ShouldCheckDataRows)
                {
                    if (this.ViewTemplate != null && !this.ViewTemplate.IsSelfReference && !this.MasterTemplate.IsSelfReference)
                    {
                        this.MasterTemplate.EndUpdate(true, new DataViewChangedEventArgs(ViewChangedAction.DataChanged));
                    }

                    return;
                }

                this.GridViewElement.EditorManager.EndEdit();

                this.TableElement.BeginUpdate();
                this.MasterTemplate.MasterViewInfo.TableSearchRow.SuspendSearch();
                List<GridViewRowInfo> list = GetRowsToIterateOver();
                foreach (GridViewRowInfo rowInfo in list)
                {
                    GridViewGroupRowInfo groupRow = rowInfo as GridViewGroupRowInfo;

                    if (groupRow != null)
                    {
                        this.CheckAllCheckBoxInChildRows(groupRow, valueState);
                    }
                    else
                    {
                        rowInfo.Cells[this.ColumnIndex].Value = valueState;
                    }
                }
                this.MasterTemplate.MasterViewInfo.TableSearchRow.ResumeSearch();
                this.TableElement.EndUpdate(false);

                if (this.ViewTemplate != null && !this.ViewTemplate.IsSelfReference && !this.MasterTemplate.IsSelfReference)
                {
                    this.MasterTemplate.EndUpdate(true, new DataViewChangedEventArgs(ViewChangedAction.DataChanged));
                }
                else
                {
                    this.TableElement.EndUpdate(false);
                }

                this.TableElement.Update(GridUINotifyAction.DataChanged);
            }

            private void CheckAllCheckBoxInChildRows(GridViewGroupRowInfo row, object state)
            {
                List<GridViewRowInfo> list = new List<GridViewRowInfo>();

                foreach (GridViewRowInfo rowInfo in row.ChildRows)
                {
                    list.Add(rowInfo);
                }

                foreach (GridViewRowInfo rowInfo in list)
                {
                    GridViewGroupRowInfo groupInfo = rowInfo as GridViewGroupRowInfo;
                    if (groupInfo != null)
                    {
                        this.CheckAllCheckBoxInChildRows(groupInfo, state);
                    }
                    else
                    {
                        rowInfo.Cells[this.ColumnIndex].Value = state;
                    }
                }
            }

            private List<GridViewRowInfo> GetRowsToIterateOver()
            {
                if (this.ViewTemplate != null && this.ViewTemplate.IsSelfReference)
                {
                    PrintGridTraverser traverser = new PrintGridTraverser(this.ViewInfo);
                    List<GridViewRowInfo> result = new List<GridViewRowInfo>();

                    while (traverser.MoveNext())
                    {
                        if (traverser.Current is GridViewDataRowInfo)
                        {
                            result.Add(traverser.Current);
                        }
                    }

                    return result;
                }

                return new List<GridViewRowInfo>(this.ViewInfo.Rows);
            }
        }
    }

Currently the time for opening the popup for a column with 100 000 unique items including blank items is about 8s, this can be optimized.

How to reproduce: 
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        this.radGridView1.EnableFiltering = true;
        this.radGridView1.ShowHeaderCellButtons = true;

        this.radGridView1.DataSource = this.GetData();
        this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;

        this.radGridView1.MouseDown += RadGridView1_MouseDown;
        this.radGridView1.FilterPopupInitialized += RadGridView1_FilterPopupInitialized;
    }

    Stopwatch sw = new Stopwatch();

    private void RadGridView1_MouseDown(object sender, MouseEventArgs e)
    {
        GridFilterButtonElement btn = this.radGridView1.ElementTree.GetElementAtPoint(e.Location) as GridFilterButtonElement;
        if (btn != null)
        {
            sw.Start();
        }
    }

    private void RadGridView1_FilterPopupInitialized(object sender, FilterPopupInitializedEventArgs e)
    {
        ((RadListFilterPopup)e.FilterPopup).PopupOpened += Form1_PopupOpened;
        
    }

    private void Form1_PopupOpened(object sender, EventArgs args)
    {
        sw.Stop();
        Console.WriteLine("Total: " + sw.ElapsedMilliseconds);
    }

    private DataTable GetData()
    {
        DataTable dt = new DataTable();
        dt.Columns.Add("Id", typeof(int));
        dt.Columns.Add("Name", typeof(string));
        dt.Columns.Add("Date", typeof(DateTime));
        dt.Columns.Add("Bool", typeof(bool));
        for (int i = 0; i < 100000; i++)
        {
            dt.Rows.Add(i, "Name " + i, DateTime.Now.AddDays(i), i % 2 == 0);
        }


        dt.Rows.Add(1, null, DateTime.Now.AddDays(1), false);
        return dt;
    }
}

How to reproduce: check also the attached video
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        this.radGridView1.DataSource = this.GetData();
        this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;

        GroupDescriptor descriptor = new GroupDescriptor();
        descriptor.GroupNames.Add("Name", ListSortDirection.Ascending);
        this.radGridView1.GroupDescriptors.Add(descriptor);
    }

    private DataTable GetData()
    {
        DataTable dt = new DataTable();
        dt.Columns.Add("Name", typeof(string));
        dt.Columns.Add("Age", typeof(int));
        dt.Columns.Add("Date", typeof(DateTime));
        dt.Columns.Add("Bool", typeof(bool));
        for (int i = 0; i < 3; i++)
        {
            for (int j = 0; j < 60; j++)
            {
                dt.Rows.Add("Name " + i, i, DateTime.Now.AddDays(i), i % 2 == 0);
            }

        }

        return dt;
    }

    private void radButton1_Click(object sender, EventArgs e)
    {
        GridPrintStyle style = new GridPrintStyle();
        style.FitWidthMode = PrintFitWidthMode.FitPageWidth;
        this.radGridView1.PrintStyle = style;

        RadPrintDocument doc = new RadPrintDocument();
        doc.HeaderHeight = 30;
        doc.HeaderFont = new Font("Arial", 12);
        doc.LeftHeader = "Left Header";
        doc.MiddleHeader = "Middle header";
        doc.RightHeader = "Right header";
        doc.AssociatedObject = this.radGridView1;

        RadPrintPreviewDialog dialog = new RadPrintPreviewDialog(doc);
        dialog.Show();
    }
}

Workaround: create a custom GridPrintStyle class
public class MyGridPrintStyle : GridPrintStyle
{
    protected override BaseGridPrintRenderer InitializePrintRenderer(RadGridView grid)
    {
        BaseGridPrintRenderer rend = base.InitializePrintRenderer(grid);
        if (rend is TableViewDefinitionPrintRenderer)
        {
            return new MyTableViewDefinitionPrintRenderer(grid);
        }

        return rend;
    }
 
}

public class MyTableViewDefinitionPrintRenderer : TableViewDefinitionPrintRenderer
{
   
    public MyTableViewDefinitionPrintRenderer(RadGridView grid)
        : base(grid)
    {
    }     

    protected override void PrintRowWideCell(GridViewRowInfo row, TableViewRowLayout rowLayout, GridPrintSettings settings, int currentX, int currentY, Graphics graphics)
    {
        int groupLevel = row.Group != null ? row.Group.Level + 1 : 0;
        int indentLevel = row.HierarchyLevel + 1 - groupLevel;
        Size cellSize = this.GetRowSize(row, rowLayout);
        int cellX = currentX + indentLevel * settings.HierarchyIndent + rowLayout.Owner.CellSpacing;
        Rectangle cellRect = new Rectangle(cellX, currentY, cellSize.Width - indentLevel * settings.HierarchyIndent, cellSize.Height);
        CellPrintElement printCell = new CellPrintElement();

        if (row is GridViewGroupRowInfo)
        {
            if (this.PrintPages.Count > 0 && !settings.PrintHierarchy)
            {
                cellRect.Width -= this.PrintPages[this.CurrentPrintPage].Count - 1;
            }

            printCell = this.CreateGroupCellPrintElement(row as GridViewGroupRowInfo);

            if (printCell.Font != settings.GroupRowFont)
            {
                if (settings.GroupRowFont != null)
                {
                    printCell.Font = settings.GroupRowFont;
                }
                else
                {
                    settings.GroupRowFont = printCell.Font;
                }
            }
        }

        printCell.TextPadding = this.GridView.PrintStyle.CellPadding;
        printCell.RightToLeft = this.GridView.RightToLeft == RightToLeft.Yes;

        PrintCellFormattingEventArgs formattEventArgs = new PrintCellFormattingEventArgs(row, null, printCell);
        this.OnPrintCellFormatting(formattEventArgs);


        PrintCellPaintEventArgs paintEventArgs = new PrintCellPaintEventArgs(graphics, row, null, cellRect);
        this.OnPrintCellPaint(paintEventArgs);

        formattEventArgs.PrintCell.Paint(graphics, paintEventArgs.CellRect);

    }
}

How to reproduce: check the attached video
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
        
        this.radGridView1.DataSource = this.GetData(1000);
        this.radGridView1.AutoExpandGroups = true;
        this.radGridView1.EnableFiltering = true;
        this.radGridView1.EnablePaging = true;
        this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
    }

    private DataTable GetData(int count)
    {
        DataTable dt = new DataTable();
        dt.Columns.Add("Id", typeof(int));
        dt.Columns.Add("Name", typeof(string));
        dt.Columns.Add("Date", typeof(DateTime));
        dt.Columns.Add("Bool", typeof(bool));

        for (int i = 0; i < count; i++)
        {

            dt.Rows.Add(i,"Name " + i,  DateTime.Now.AddDays(i), i % 2 == 0 ? true : false);
        }

        return dt;
    }
}

Workaround: cancel the PageChanging event
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
        
        this.radGridView1.DataSource = this.GetData(1000);
        this.radGridView1.AutoExpandGroups = true;
        this.radGridView1.EnableFiltering = true;
        this.radGridView1.EnablePaging = true;
        this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;

        timer = new Timer();
        timer.Interval = 100;
        timer.Tick += (sender, e) =>
        {
            timer.Stop();
            this.shouldCancel = false;
        };

        this.radGridView1.PageChanging += RadGridView1_PageChanging;
        this.radGridView1.CurrentRowChanged += RadGridView1_CurrentRowChanged;
    }

    private void RadGridView1_CurrentRowChanged(object sender, CurrentRowChangedEventArgs e)
    {
        this.shouldCancel = this.ShouldCancelPageChange(e.CurrentRow);
        timer.Start();
    }

    Timer timer;
    bool shouldCancel = false;

    private bool ShouldCancelPageChange(GridViewRowInfo rowInfo)
    {
        if (this.radGridView1.TableElement.MasterTemplate != null && this.radGridView1.TableElement.MasterTemplate.EnablePaging)
        {
            int pageIndex = this.radGridView1.TableElement.ViewTemplate.DataView.GetItemPage(rowInfo);

            if (pageIndex == this.radGridView1.TableElement.MasterTemplate.PageIndex)
            {
                return true;
            }
        }

        return false;
    }

    private void RadGridView1_PageChanging(object sender, Telerik.WinControls.PageChangingEventArgs e)
    {
        e.Cancel = this.shouldCancel;
    }

    private DataTable GetData(int count)
    {
        DataTable dt = new DataTable();
        dt.Columns.Add("Id", typeof(int));
        dt.Columns.Add("Name", typeof(string));
        dt.Columns.Add("Date", typeof(DateTime));
        dt.Columns.Add("Bool", typeof(bool));

        for (int i = 0; i < count; i++)
        {

            dt.Rows.Add(i,"Name " + i,  DateTime.Now.AddDays(i), i % 2 == 0 ? true : false);
        }

        return dt;
    }
}

I have a GridView (Winforms v. 2017.3.1017.40, same problem with 2017.2.502.40) configured as on the attached screenshot (made some groups, made a search, and used the "excel-like" filter on a column to uncheck some values).

Then I try to click on some column headers to sort the grid.
After a few clicks, I get this error :
InvalidOperationException: La collection a été modifiée ; l'opération d'énumération peut ne pas s'exécuter.

I got different stacktrace with the same error :
InvalidOperationException: La collection a été modifiée ; l'opération d'énumération peut ne pas s'exécuter.
   at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
   at System.Collections.Generic.List`1.Enumerator.MoveNextRare()
   at System.Collections.Generic.List`1.Enumerator.MoveNext()
   at Telerik.WinControls.UI.DataGroup.<GetEnumerator>d__0.MoveNext()
   at Telerik.WinControls.UI.GridViewGroupRowInfo.get_ChildRows()
   at Telerik.WinControls.UI.GridViewRowInfo.HasChildRows()
   at Telerik.WinControls.UI.PrintGridTraverser.CanStepInHierarchy()
   at Telerik.WinControls.UI.GridTraverser.StepInHierarchy()
   at Telerik.WinControls.UI.GridTraverser.MoveNextCore()
   at Telerik.WinControls.UI.GridTraverser.MoveNext()
   at Telerik.WinControls.UI.GridViewSearchRowInfo.worker_DoWork(Object sender, DoWorkEventArgs e)
   at System.ComponentModel.BackgroundWorker.OnDoWork(DoWorkEventArgs e)
   at System.ComponentModel.BackgroundWorker.WorkerThreadStart(Object argument)

-----

InvalidOperationException: Failed to compare two elements in the array. ---> NullReferenceException: Object reference not set to an instance of an object.
   at Telerik.WinControls.UI.GridViewRowInfoComparer.CompareRows(GridViewRowInfo x, GridViewRowInfo y, SortDescriptorCollection context)
   at System.Collections.Generic.ArraySortHelper`1.InternalBinarySearch(T[] array, Int32 index, Int32 length, T value, IComparer`1 comparer)
   at System.Collections.Generic.ArraySortHelper`1.BinarySearch(T[] array, Int32 index, Int32 length, T value, IComparer`1 comparer)


   --- End of inner exception stack trace ---
   at System.Collections.Generic.ArraySortHelper`1.BinarySearch(T[] array, Int32 index, Int32 length, T value, IComparer`1 comparer)
   at System.Collections.Generic.List`1.BinarySearch(Int32 index, Int32 count, T item, IComparer`1 comparer)
   at Telerik.Collections.Generic.HybridIndex`1.PerformWithQuickSort()
   at Telerik.Collections.Generic.HybridIndex`1.get_Items()
   at Telerik.Collections.Generic.Index`1.GetEnumerator()
   at Telerik.WinControls.Data.GroupBuilder`1.Perform(IReadOnlyCollection`1 items, Int32 level, Group`1 parent)
   at Telerik.WinControls.Data.GroupBuilder`1.get_Groups()
   at Telerik.WinControls.UI.GridViewInfo.Refresh()
   at Telerik.WinControls.UI.GridViewInfo.get_ChildRows()
   at Telerik.WinControls.UI.ViewInfoTraverser.SetCollectionForStage(Boolean initializeCollection)
   at Telerik.WinControls.UI.ViewInfoTraverser.ChangeCollectionForward()
   at Telerik.WinControls.UI.ViewInfoTraverser.MoveNextCore()
   at Telerik.WinControls.UI.ViewInfoTraverser.MoveNext()
   at Telerik.WinControls.UI.GridTraverser.MoveNextCore()
   at Telerik.WinControls.UI.GridTraverser.MoveNext()
   at Telerik.WinControls.UI.BaseGridNavigator.NavigateToRow(GridViewRowInfo row, Boolean select)
   at Telerik.WinControls.UI.BaseGridNavigator.ProcessViewChangedEvent(GridViewEvent eventData)
   at Telerik.WinControls.UI.BaseGridNavigator.ProcessEventCore(GridViewEvent eventData)
   at Telerik.WinControls.UI.GridViewEventProcessEntity.ProcessCollection(GridViewEvent gridEvent, PriorityWeakReferenceList list, GridEventProcessMode processMode)
   at Telerik.WinControls.UI.GridViewEventProcessEntity.ProcessEvent(GridViewEvent gridEvent)
   at Telerik.WinControls.UI.GridViewSynchronizationService.NotifyListeners(GridViewEvent gridEvent)
   at Telerik.WinControls.UI.GridViewSynchronizationService.FlushEvents()
   at Telerik.WinControls.UI.GridViewSynchronizationService.DispatchEvent(GridViewEvent gridEvent)
   at Telerik.WinControls.UI.GridViewSynchronizationService.DispatchEvent(GridViewTemplate template, GridViewEvent eventData, Boolean postUI)
   at Telerik.WinControls.UI.GridViewTemplate.DispatchDataViewChangedEvent(Object sender, DataViewChangedEventArgs args)
   at Telerik.WinControls.UI.GridViewTemplate.OnViewChanged(Object sender, DataViewChangedEventArgs e)
   at Telerik.WinControls.UI.MasterGridViewTemplate.OnViewChanged(Object sender, DataViewChangedEventArgs e)
   at Telerik.WinControls.Data.RadDataView`1.RebuildData(Boolean notify)
   at Telerik.WinControls.Data.RadDataView`1.OnNotifyPropertyChanged(PropertyChangedEventArgs e)
   at Telerik.WinControls.Data.RadCollectionView`1.OnNotifyPropertyChanged(String propertyName)
   at Telerik.WinControls.UI.GridViewSortDescriptorCollection.OnCollectionChanged(NotifyCollectionChangedEventArgs args)
   at System.ComponentModel.PropertyChangedEventHandler.Invoke(Object sender, PropertyChangedEventArgs e)
   at Telerik.WinControls.Data.SortDescriptor.set_Direction(ListSortDirection value)
   at Telerik.WinControls.UI.GridViewColumn.Sort(RadSortOrder sortOrder, Boolean multiSortMode)
   at Telerik.WinControls.UI.GridHeaderCellElement.Sort(RadSortOrder sortOrder)
   at Telerik.WinControls.UI.GridHeaderRowBehavior.OnMouseUp(MouseEventArgs e)
   at Telerik.WinControls.UI.BaseGridBehavior.OnMouseUp(MouseEventArgs e)
   at Telerik.WinControls.UI.RadGridView.OnMouseUp(MouseEventArgs e)
   at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at Telerik.WinControls.RadControl.WndProc(Message& m)
   at Telerik.WinControls.UI.RadGridView.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

------

InvalidOperationException: Collection was modified; enumeration operation may not execute.
   at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
   at System.Collections.Generic.List`1.Enumerator.MoveNextRare()
   at System.Collections.Generic.List`1.Enumerator.MoveNext()
   at Telerik.WinControls.UI.DataGroup.<GetEnumerator>d__0.MoveNext()
[...]
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at Alto.Client.Controls.Dispatch.DispatchResources.DeliveryPointFilterManager.get_DeliveryPointsVisible() in d:\Builds\45153\Alto\Alto.Prod\src\Alto\Alto.Client\Controls\Dispatch\DispatchResources\DeliveryPointFilterManager.cs:line 38
   at Alto.Client.Controls.Dispatch.DispatchResources.UcDeliveryPoint.get_Title() in d:\Builds\45153\Alto\Alto.Prod\src\Alto\Alto.Client\Controls\Dispatch\DispatchResources\UcDeliveryPoint.cs:line 1520
   at Alto.Client.Controls.Dispatch.DispatchResources.UcDeliveryPoint.updatePanelTitle() in d:\Builds\45153\Alto\Alto.Prod\src\Alto\Alto.Client\Controls\Dispatch\DispatchResources\UcDeliveryPoint.cs:line 543
   at Alto.Client.Controls.Dispatch.DispatchResources.UcDeliveryPoint.onFilterChanged(Object sender, GridViewCollectionChangedEventArgs e) in d:\Builds\45153\Alto\Alto.Prod\src\Alto\Alto.Client\Controls\Dispatch\DispatchResources\UcDeliveryPoint.cs:line 547
   at Telerik.WinControls.UI.GridViewCollectionChangedEventHandler.Invoke(Object sender, GridViewCollectionChangedEventArgs e)
   at Telerik.WinControls.UI.RadGridView.OnFilterChanged(Object sender, GridViewCollectionChangedEventArgs e)
   at Telerik.WinControls.UI.EventDispatcher.RaiseEvent[T](Object eventKey, Object sender, T args)
   at Telerik.WinControls.UI.GridViewFilterDescriptorCollection.OnCollectionChanged(NotifyCollectionChangedEventArgs args)
   at Telerik.Collections.Generic.NotifyCollection`1.InsertItem(Int32 index, T item)
   at Telerik.WinControls.UI.GridViewFilterDescriptorCollection.InsertItem(Int32 index, FilterDescriptor item)
   at System.Collections.ObjectModel.Collection`1.Add(T item)
   at Telerik.WinControls.UI.GridViewDataColumn.SetFilterDescriptor(FilterDescriptor value)
   at Telerik.WinControls.UI.GridHeaderCellElement.filterPopup_FilterConfirmed(Object sender, EventArgs e)
   at Telerik.WinControls.UI.BaseFilterPopup.OnFilterConfirmed()
   at Telerik.WinControls.UI.RadListFilterPopup.OnButtonOkClick(EventArgs e)
   at Telerik.WinControls.UI.RadListFilterPopup.ButtonOK_Click(Object sender, EventArgs e)
   at Telerik.WinControls.RadElement.OnClick(EventArgs e)
   at Telerik.WinControls.UI.RadButtonItem.OnClick(EventArgs e)
   at Telerik.WinControls.UI.RadButtonElement.OnClick(EventArgs e)
   at Telerik.WinControls.RadElement.DoClick(EventArgs e)
   at Telerik.WinControls.RadElement.RaiseBubbleEvent(RadElement sender, RoutedEventArgs args)
   at Telerik.WinControls.RadItem.RaiseBubbleEvent(RadElement sender, RoutedEventArgs args)
   at Telerik.WinControls.RadElement.RaiseRoutedEvent(RadElement sender, RoutedEventArgs args)
   at Telerik.WinControls.RadElement.RaiseBubbleEvent(RadElement sender, RoutedEventArgs args)
   at Telerik.WinControls.RadItem.RaiseBubbleEvent(RadElement sender, RoutedEventArgs args)
   at Telerik.WinControls.RadElement.RaiseRoutedEvent(RadElement sender, RoutedEventArgs args)
   at Telerik.WinControls.RadElement.DoMouseUp(MouseEventArgs e)
   at Telerik.WinControls.ComponentInputBehavior.OnMouseUp(MouseEventArgs e)
   at Telerik.WinControls.RadControl.OnMouseUp(MouseEventArgs e)
   at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.ScrollableControl.WndProc(Message& m)
   at Telerik.WinControls.RadControl.WndProc(Message& m)
   at Telerik.WinControls.UI.RadPopupControlBase.WndProc(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

----

InvalidOperationException: Collection was modified; enumeration operation may not execute.
   at System.Collections.Generic.List`1.Enumerator.MoveNextRare()
   at Telerik.WinControls.UI.DataGroup.<GetEnumerator>d__0.MoveNext()
   at Telerik.WinControls.Data.GroupBuilder`1.Perform(IReadOnlyCollection`1 items, Int32 level, Group`1 parent)
   at Telerik.WinControls.Data.DataItemGroup`1.get_Groups()
   at Telerik.WinControls.UI.DataGroup.get_Groups()
   at Telerik.WinControls.UI.GridViewTemplate.InvalidateSummaryRowsCore(DataGroup group, Boolean invalidateChildren)
   at Telerik.WinControls.UI.GridViewTemplate.InvalidateSummaryRowsCore(DataGroup group, Boolean invalidateChildren)
   at Telerik.WinControls.UI.GridViewTemplate.InvalidateGroupSummaryRows(DataGroup group, Boolean invalidateChildren)
   at Telerik.WinControls.UI.GridViewTemplate.RefreshSummaryRowsInGroup(NotifyCollectionChangedEventArgs e)
   at Telerik.WinControls.UI.GridViewTemplate.RefreshAggregates(NotifyCollectionChangedEventArgs e)
   at Telerik.WinControls.UI.MasterGridViewTemplate.OnViewChanged(Object sender, DataViewChangedEventArgs e)
   at Telerik.WinControls.Data.RadDataView`1.RebuildData(Boolean notify)
   at Telerik.WinControls.Data.RadDataView`1.OnNotifyPropertyChanged(PropertyChangedEventArgs e)
   at Telerik.WinControls.Data.RadCollectionView`1.OnNotifyPropertyChanged(String propertyName)
   at Telerik.WinControls.UI.GridViewSortDescriptorCollection.OnCollectionChanged(NotifyCollectionChangedEventArgs args)
   at Telerik.WinControls.UI.GridViewSortDescriptorCollection.RemoveItem(Int32 index)
   at Telerik.WinControls.UI.GridViewColumn.Sort(RadSortOrder sortOrder, Boolean multiSortMode)
   at Telerik.WinControls.UI.GridHeaderCellElement.Sort(RadSortOrder sortOrder)
   at Telerik.WinControls.UI.GridHeaderRowBehavior.OnMouseUp(MouseEventArgs e)
   at Telerik.WinControls.UI.BaseGridBehavior.OnMouseUp(MouseEventArgs e)
   at Telerik.WinControls.UI.RadGridView.OnMouseUp(MouseEventArgs e)
   at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at Telerik.WinControls.RadControl.WndProc(Message& m)
   at Telerik.WinControls.UI.RadGridView.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

To reproduce: please refer to the attached gif file and sample project.

Workaround:

        private void radGridView1_CellEndEdit(object sender, GridViewCellEventArgs e)
        {
            RadDropDownListEditor editor = e.ActiveEditor as RadDropDownListEditor;
            if (editor!=null)
            {
                RadDropDownListEditorElement el = editor.EditorElement as RadDropDownListEditorElement;
                el.AutoCompleteSuggest.DropDownList.ClosePopup();
            } 
        }

To reproduce:

   Sub New()
         
        InitializeComponent()
        Dim dt As New DataTable
        dt.Columns.Add("Id", GetType(Integer))
        dt.Columns.Add("Name", GetType(String))
        For index = 1 To 1000
            dt.Rows.Add(index, "Item" & index)
        Next
        Me.RadGridView1.DataSource = dt


    End Sub
    Private Sub RadButton1_Click(sender As Object, e As EventArgs) Handles RadButton1.Click
        Dim pdfFile As String = "..\..\exportedFile.pdf"
        Dim pdfExporter As New GridViewPdfExport(Me.RadGridView1)
        pdfExporter.ShowHeaderAndFooter = True
        pdfExporter.FitToPageWidth = True
        AddHandler pdfExporter.PdfExported, AddressOf pdfExporter_PdfExported
        Dim renderer As New PdfExportRenderer()
        pdfExporter.RunExport(pdfFile, renderer)

        Process.Start(pdfFile)
    End Sub

    Private Sub pdfExporter_PdfExported(sender As Object, e As System.EventArgs)

        Dim pdfFile As String = "..\..\exportedFile.pdf"
        Dim document As RadFixedDocument
        Dim provider As Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.PdfFormatProvider = New Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.PdfFormatProvider()
        Using stream As Stream = File.OpenRead(pdfFile)
            document = provider.Import(stream)  '<==== Error Found
            For Each page As RadFixedPage In document.Pages
                Dim editor As FixedContentEditor = New FixedContentEditor(page)
                editor.Position.Translate(page.Size.Width / 2, page.Size.Height - 50)

                Dim pageNum As Integer = document.Pages.IndexOf(page) + 1
                editor.DrawText(pageNum + " of " + document.Pages.Count)
            Next
        End Using

        Using output As Stream = File.OpenWrite(pdfFile)
            provider.Export(document, output)    '<==== Error Found
        End Using

        Process.Start(pdfFile)
    End Sub

Workaround: use the PdfExportRenderer.PdfExporting event where you have access to the document and you can make any customizations to it:
        Dim pdfFile As String = "..\..\exportedFile.pdf"
        Dim pdfExporter As New GridViewPdfExport(Me.RadGridView1)
        pdfExporter.ShowHeaderAndFooter = True
        pdfExporter.FitToPageWidth = True
        Dim renderer As New PdfExportRenderer()
        AddHandler renderer.PdfExporting, AddressOf PdfExporting
        pdfExporter.RunExport(pdfFile, renderer)

    Private Sub PdfExporting(sender As Object, e As PdfExportingEventArgs)
        Dim document As RadFixedDocument = e.Document
        Dim provider As Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.PdfFormatProvider = New Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.PdfFormatProvider()

        For Each page As RadFixedPage In document.Pages
            Dim editor As FixedContentEditor = New FixedContentEditor(page)
            editor.Position.Translate(page.Size.Width / 2, page.Size.Height - 50)

            Dim pageNum As Integer = document.Pages.IndexOf(page) + 1
            editor.DrawText(pageNum & " of " & document.Pages.Count)
        Next 
    End Sub