Oliver, I've just answered your ticket on the same subject. For convenience I am pasting my reply below:
Can you please verify that you have followed all steps in the following Help article: http://www.telerik.com/help/radeditormoss/?Using%20RadEditor%20in%20List%20Items.html, including the last one - resetting the IIS? I am asking you this because this is the most likely reason for this problem. If you did this, can you please make sure that you are using an administrator account when you set up the editor?
the Telerik team