You have indicated in you support ticket that you are still using the older version 5.8.2 (2010.2.929) of RadEditor for MOSS. Could you please confirm that this is the version that you are using? I checked our records and saw that you have also downloaded the latest version 5.8.10. Have your tried it and see whether the reported problem is fixed? If not, my recommendation is to test it, because the problem could be fixed in it.
You can find migration instructions in the following help article: Upgrading to a newer version of RadEditor for MOSS
If the problem still persists after the upgrade please provide a live URL reproducing the problem?
This error might occur if some of the results of some of RadFileExplorer's AJAX calls is corrupted. To be able to provide more to the point answer we will need to examine and debug the issue.
If you cannot provide a live URL, could you please provide a fiddler log captured the problematic scenario? You can find information how to work with this useful tool at: http://www.west-wind.com/weblog/posts/596348.aspx
the Telerik team