To reproduce:
Open a form with a ribbon, focus the tabs and press 1 with the screen keyboard (German language). 

Workaround:
class MyRibbon : RadRibbonBar
{
    protected override ComponentInputBehavior CreateBehavior()
    {
   
        return new MyComponentBehavior(this);
    }
    public override string ThemeClassName
    {
        get { return typeof(RadRibbonBar).FullName; }
    }
}
class MyComponentBehavior : RibbonBarInputBehavior
{
    public MyComponentBehavior(RadRibbonBar owner) : base(owner)
    {

    }
    protected override string GetKeyStringRepresentation(Keys input)
    {
        uint nonVirtualKey = NativeMethods.MapVirtualKey((uint)input, 2);

        if (nonVirtualKey > char.MaxValue)
        {
            return null;
        }

        return base.GetKeyStringRepresentation(input);
    }
}