Follow the steps:

1- Add RadRibbonBar1 to the form. The program compiles successfully.
2- Add RibbonTab1 to the RibbonBar. The program compiles successfully.
3. Add RadRibbonBarGroup1. The program does not compile and the mentioned error appears.

Error 1 Invalid Resx file. Could not load type System.DelegateSerializationHolder, Telerik.WinControls.UI, Version=2022.2.622.40, Culture=neutral, PublicKeyToken=5bb2a467cbec794e which is used in the .RESX file.  Ensure that the necessary references have been added to your project. Line 142, position 5. C:\Projects\1570764TestRibbonResx\1570764TestRibbonResx\RadForm1.resx 142 5 1570764TestRibbonResx

Metro theme for RadRibborBar has glitches for the application menu and quick access bar.

FIX. RadRibbonBar - the Click event of the OptionsButton is fired twice

Steps to reproduce.

1. Add a form with a RadRibbonBar 
2. Add a RadCheckBox to the ribbon
3. Set the IsThreeState property of the check box to true
4. Run the project and click through the states of the check box. You will see that the checked and indeterminate states are looking the same way.

ADD. RadRibbonBar - add cancelable CommandTabChanging event.
The event is called CommandTabSelecting to be coherent with the older event CommandTabSelected.

It will be nice if RadRibbonBar has the QuickAccessToolbar property exposed in the property grid of Visual Studio, just like ExpandButton, ExitButton, etc.

To reproduce:
1.Add a RadRibbonBar (with several groups and buttons inside a group) and a theme (i.e. TelerikMetro).
2.Apply the theme for the RadRibbonBar: setting the ThemeName property to "YourThemeName".
for example: this.radRibbonBar1.ThemeName = "TelerikMetro";
3. Run the application and resize the form in a way to collapse all ribbon groups. When you open the popup for the group, all popup elements are with incorrect style.

Workaround: load the theme in the form's constructor.

To reproduce:
- Add a ribbon bar to a blank form and set the theme to Office2013Light
- Disable the title bar buttons:
this.radRibbonBar1.MaximizeButton = false;
this.radRibbonBar1.MinimizeButton = false;
this.radRibbonBar1.CloseButton = false;

- The buttons are still visible when the backstage view button is clicked.

Workaround:
radRibbonBar1.BackstageControl.BackstageElement.TitleBarElement.CloseButton.Visibility = Telerik.WinControls.ElementVisibility.Hidden;
radRibbonBar1.BackstageControl.BackstageElement.TitleBarElement.MaximizeButton.Visibility = Telerik.WinControls.ElementVisibility.Hidden;
radRibbonBar1.BackstageControl.BackstageElement.TitleBarElement.MinimizeButton.Visibility = Telerik.WinControls.ElementVisibility.Hidden;

To reproduce:

Create a RadRibbonForm and set its ApplicationMenuStyle to BackstageView. Start the application open the backstage, click the maximize button and then again to set the Form back to normal. You will notice that the Form cannot be set back to normal.

Workaround:

Manually change the WindowState of the Form.

this.RibbonBar.BackstageControl.BackstageElement.TitleBarElement.MaximizeButton.MouseDown +=MaximizeButton_Click;

void MaximizeButton_Click(object sender, EventArgs e)
        {
            if (this.WindowState == FormWindowState.Maximized)
            {
                this.WindowState = FormWindowState.Normal;
            }
            else if (this.WindowState == FormWindowState.Normal)
            {
                this.WindowState = FormWindowState.Maximized;
            }
        }

To reproduce:

public Form1()
{
    InitializeComponent();
    this.AllowAero = false;
    this.RibbonBar.Expanded = false;
    ThemeResolutionService.ApplicationThemeName = "TelerikMetro"; 
}

To reproduce:
- Press and hold right mouse button.
- Move mouse cursor outside bounds of the button.
- Release mouse button - the button fill is not correct.

Please refer to the attached gif file.

Workaround: adjust manually the RadRibbonBarGroup.Margin when the expanded state is changed:

public Form1()
{
    InitializeComponent();
    this.radRibbonBar1.ExpandedStateChanged += radRibbonBar1_ExpandedStateChanged;
    radRibbonBar1.Expanded = false;
}

private void radRibbonBar1_ExpandedStateChanged(object sender, EventArgs e)
{
    if (this.radRibbonBar1.Expanded)
    {
        AdjustMargin(0);
    }
    else
    {
        AdjustMargin(-10);
    }
}

private void AdjustMargin(int top)
{
    foreach (RibbonTab tab in this.radRibbonBar1.CommandTabs)
    {
        foreach (RadRibbonBarGroup group in tab.Items)
        {
            group.Margin = new Padding(0, top, 0, 0);
        }
    }
}

To reproduce:
- allow selection by setting ItemSelection = true
- set the SelectedItem to some of the elements
- set the SelectedItem to null -> the item remains selected

To reproduce: 
1. Add a RadRibbonBar and a RadButtonElement in one of the groups.
2. Set the RadButtonElement.UseCompatibleTextRendering property is set to false.
3. Set the RadButtonElement.Enabled property to false.

You will notice that the text is hardly read compared to version Q2 2014.

Workaround: Currently, the possible solution is to set the RadButtonElement.UseDefaultDisabledPaint property to false and subscribe to the EnabledChanged event where you can modify the ForeColor in order to make it gray when the button is disabled and apply a gray scaled image. 

Work around: 
 void radRibbonBar1_CommandTabSelecting(object sender, Telerik.WinControls.UI.CommandTabSelectingEventArgs args)
        {
            if (args.NewCommandTab.Visibility == Telerik.WinControls.ElementVisibility.Collapsed || args.NewCommandTab.Visibility == Telerik.WinControls.ElementVisibility.Hidden)
            {
                args.Cancel = true;
            }
        }

Please refer to the attached gif file.

Workaround: close manually the popup by calling the this.radSplitButtonElement1.DropDownMenu.ClosePopup(Telerik.WinControls.UI.RadPopupCloseReason.Mouse) method.

To reproduce:
Add RadRibbonForm and change the theme(for example to WIndows8Theme). Run the project. The system buttons are covered by the fill of RadRibbonBarElement.

Workaround:
Set the DrawFill property of RadRibbonBarElement to false.
this.RibbonBar.RibbonBarElement.DrawFill = false;

Use the attached project to reproduce.

Workaround:
private void RadForm1_SizeChanged(object sender, EventArgs e)
{
    if (this.WindowState == FormWindowState.Normal)
    {
        this.SizeChanged -= RadForm1_SizeChanged;
        this.Size = new Size(this.Size.Width + 1, this.Size.Height + 1);
        this.Size = new Size(this.Size.Width - 1, this.Size.Height - 1);

        this.SizeChanged += RadForm1_SizeChanged;
    }
}

To reproduce:

1. Collapse the ribbon
2. Select the tab to show the popup
3. Type some text in the RadDropDownListElement
4. Press Escape, Enter, Backspace keys. You will notice that these keys are not handled by the drop down. However, the ribbon popup is closed and there is no way to handle these keys.