The problem could be due to an incorrect path or to that you try to access a folder under a different sitecollection. You can access the folders only in the current site collection.
Could you please send the ConfigFile.xml / ListConfigFile.xml so that I can see how the paths are set?
These configuration files are located in the \Program Files\Common Files\Microsoft Shared\web server extensions\wpresources\RadEditorSharePoint\5.x.xx.0__1f131a624888eeed\Resources folder, where 5.x.xx numbers represent the RadEditor for MOSS version.
You can find more information in the following help article: Set Properties Via Config File
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed