When the control is used in NoXaml scenario and the required resources are merged in the resources of the Window/UserControl containing RadRichTextBox, XamlParseException is thrown at runtime with a message "Cannot find resource named 'thumbStroke'."
or
the App.Current.Resources.MergedDictionaries is cleared.

Workaround: Merge Telerik.Windows.Documents.xaml and Telerik.Windows.Controls.RichTextBoxUI.xaml in App.xaml.
or
Merge the resourced before initializing RadRichTextBox:
 App.Current.Resources.MergedDictionaries.Add(new ResourceDictionary()
            {
                Source = new Uri("/Telerik.Windows.Documents;component/Themes/Styles/ImageAdornerUIResourcesDictionary.xaml", UriKind.RelativeOrAbsolute)
            });


Fix available in R3 2017 Official Release version.