LocalizableException with text "Format string is not in the correct format.", with inner ArgumentException "Format string is not in the correct format." is thrown. 

Currently the validation doesn't allow number format string starting with date or time formatting to have more that one section (for negative and zero numbers). Excel allows this type of number formats and if you set format like dd/mm/yyyy;[RED](#.##0); the positive numbers are visualized as dates, but negative numbers are visualized as numbers and their fore color is red.

Examples (including the final semicolons):

- ss;

- dd;

- dd/mm/yyyy;;

- mmm-yy;mmm-yy;_-* "-"??_-;_-@_-

RadSpreadsheetFormulaBar has a drop down list that shows the named ranges in the document. If a named range contains an underscore character (ex: Income_Amount), the associated item in the drop down won't display the underscore (ex: IncomeAmount).

To workaround this, you can use a global Loaded event handler of RadMenuItem and override the Header content with a TextBlock.

static MainWindow()
{
    EventManager.RegisterClassHandler(typeof(RadMenuItem), RadMenuItem.LoadedEvent, new RoutedEventHandler(OnMenuItemLoaded));
}

private static void OnMenuItemLoaded(object sender, RoutedEventArgs e)
{
    var menuItem = (RadMenuItem)sender;
    if (menuItem.DataContext is DefinedName context)
    {
        menuItem.Header = new TextBlock() { Text = context.Name };
    }
}