Do you use RadEditor for MOSS or the regular RadEditor for ASP.NET AJAX with custom handler / custom content provider in your MOSS site?
My recommendation is to use RadEditor for MOSS in SharePoint scenarios, because it comes with a special File Browser Content Provider designed to work with the SharePoint libraries. You can see how to configure it in the following article: Image Manager
If you are already using RadEditor for MOSS, please open a support ticket and send your configuration files placed in the \Program Files\Common Files\Microsoft Shared\web server extensions\wpresources\RadEditorSharePoint\5.x.xx.0__1f131a624888eeed\Resources
folder as well as the web.config file of the SharePoint site.
All the best,
the Telerik team