I hit this and didn't find this ticket when searching so i had filed a new ticket. Here is the information from my ticket:

It appears that if a cell has a conditional formatting rule of the type "Format only cells that contain" there is a null reference exception that occurs when you try to copy that cell. I've attached an excel sheet that uses "Cell value equal to 0" but you can change it to any of the other rules such as greater than, between, etc and the result is the same.

I am using .net 7. I originally hit it with Telerik.UI.for.Wpf.70.Xaml.2023.3.1114. I updated to Telerik.UI.for.Wpf.70.Xaml.2024.1.423 to see if it was fixed but it still appears to happen.

Simple code for repro:
using var fileStream = File.Open( spreadsheetFilePath, FileMode.Open );

var formatProvider = new Telerik.Windows.Documents.Spreadsheet.FormatProviders.OpenXml.Xlsx.XlsxFormatProvider();

using var workbook = formatProvider.Import( fileStream ); 

using var sheet1 = workbook.Worksheets.FirstOrDefault( ws => ws.Name.Equals("Sheet1", StringComparison.OrdinalIgnoreCase));

if( sheet1 != null)
{
    var test = sheet1.UsedCellRange;
    var testSelection = sheet1.Cells.GetCellSelection( test );
    testSelection.Copy();
}

Exception details:

System.NullReferenceException
  HResult=0x80004003
  Message=Object reference not set to an instance of an object.
  Source=Telerik.Windows.Documents.Spreadsheet
  StackTrace:
   at Telerik.Windows.Documents.Spreadsheet.Model.ConditionalFormattings.ConditionalFormattingDxfRule.TranslateArgument(ICellValue argument, Worksheet worksheet, CellIndex targetIndex)

The debugger shows that the argument parameter is null. Below is the call stack from the debugger since the exception call stack doesn't give the full stack.

  Telerik.Windows.Documents.Spreadsheet.dll!Telerik.Windows.Documents.Spreadsheet.Model.ConditionalFormattings.ConditionalFormattingDxfRule.TranslateArgument(Telerik.Windows.Documents.Spreadsheet.Model.ICellValue argument, Telerik.Windows.Documents.Spreadsheet.Model.Worksheet worksheet, Telerik.Windows.Documents.Spreadsheet.Model.CellIndex targetIndex)
> Telerik.Windows.Documents.Spreadsheet.dll!Telerik.Windows.Documents.Spreadsheet.Model.ConditionalFormattings.EqualToRule.CopyOverride(Telerik.Windows.Documents.Spreadsheet.Model.Worksheet targetWorksheet, Telerik.Windows.Documents.Spreadsheet.Model.CellIndex targetIndex)
  Telerik.Windows.Documents.Spreadsheet.dll!Telerik.Windows.Documents.Spreadsheet.Model.ConditionalFormattings.ConditionalFormattingDxfRule.Copy(Telerik.Windows.Documents.Spreadsheet.Model.Worksheet targetWorksheet, Telerik.Windows.Documents.Spreadsheet.Model.CellIndex targetIndex)
  Telerik.Windows.Documents.Spreadsheet.dll!Telerik.Windows.Documents.Spreadsheet.Model.ConditionalFormattings.ConditionalFormatting.Copy(Telerik.Windows.Documents.Spreadsheet.Model.Worksheet worksheet, Telerik.Windows.Documents.Spreadsheet.Model.CellIndex targetIndex)
  Telerik.Windows.Documents.Spreadsheet.dll!Telerik.Windows.Documents.Spreadsheet.Model.WorksheetFragment.CopyPasteConditionalFormattings(Telerik.Windows.Documents.Spreadsheet.Model.CopyPasteContext context, System.Collections.Generic.HashSet<Telerik.Windows.Documents.Spreadsheet.PropertySystem.IPropertyDefinition> nonDefaultProperties, Telerik.Windows.Documents.Spreadsheet.Core.DataStructures.LongRange fromLongRange, Telerik.Windows.Documents.Spreadsheet.Core.DataStructures.LongRange toLongRange)
  Telerik.Windows.Documents.Spreadsheet.dll!Telerik.Windows.Documents.Spreadsheet.Model.WorksheetFragment.CopyPasteCellProperties(Telerik.Windows.Documents.Spreadsheet.Model.CopyPasteContext context, Telerik.Windows.Documents.Spreadsheet.Core.DataStructures.LongRange fromLongRange, Telerik.Windows.Documents.Spreadsheet.Core.DataStructures.LongRange toLongRange)
  Telerik.Windows.Documents.Spreadsheet.dll!Telerik.Windows.Documents.Spreadsheet.Model.WorksheetFragment.CopyPasteCellProperties(Telerik.Windows.Documents.Spreadsheet.Model.CopyPasteContext context)
  Telerik.Windows.Documents.Spreadsheet.dll!Telerik.Windows.Documents.Spreadsheet.Model.WorksheetFragment.CopyPaste(Telerik.Windows.Documents.Spreadsheet.Model.CopyPasteContext context, out System.Collections.Generic.List<Telerik.Windows.Documents.Spreadsheet.Model.CellRange> affectedCellRanges, out System.Collections.Generic.IEnumerable<Telerik.Windows.Documents.Spreadsheet.Model.Shapes.FloatingShapeBase> newShapes)
  Telerik.Windows.Documents.Spreadsheet.dll!Telerik.Windows.Documents.Spreadsheet.Model.WorksheetFragment.WorksheetFragment(Telerik.Windows.Documents.Spreadsheet.Model.CellRange cellRange, System.Collections.Generic.IEnumerable<Telerik.Windows.Documents.Spreadsheet.Model.Shapes.FloatingShapeBase> shapes, Telerik.Windows.Documents.Spreadsheet.Model.Worksheet fromWorksheet, bool shouldPasteAsText, bool isCellCopyPaste, Telerik.Windows.Documents.Spreadsheet.Model.PasteOptions pasteOptions)
  Telerik.Windows.Documents.Spreadsheet.dll!Telerik.Windows.Documents.Spreadsheet.Model.CellSelection.CopyInternal(Telerik.Windows.Documents.Spreadsheet.Model.PasteOptions pasteOptions)
  Telerik.Windows.Documents.Spreadsheet.dll!Telerik.Windows.Documents.Spreadsheet.Model.CellSelection.Copy()