Could you please confirm that you are using the latest version of RadEditor for MOSS, which now is 5.4.1. If not, please upgrade to it. You can find instructions in the following help article http://www.telerik.com/help/aspnet-ajax/upgrade_to_a_newer_version_of_radeditor_for_moss.html
If you experience the problem after the upgrade we will be glad to help you further.
the Telerik team