In the current release the default implicit style (office black) file for RichtTextBox has errors (Telerik.Windows.Controls.RichTextBoxUI.xaml)
For example these keys are used before they are defined:
<Thickness x:Key="WindowButtonMargin">1 1 0 1</Thickness>
To be safe I copied about 20-30 lines of these declarations to the start.
After that my application works again.