I have managed to create a workaround to get around the problem.
This is really a hack, but it works :-)
So, in the loaded event of the editor, I set some content. Then I create and set a timer to do a callback after 10ms I clear the textbox. After this, everything seems to work just fine.
Thought this could be of interest for you..
Here's the code behind on the page hosting the RadRichTextBox:
Regards, Øivind Solberg