If there is an ASP.NET 2.0 unhandled exception, it should be available in the server's event viewer. Open the Application event log and look for a ASP.NET warning after you see the error message in the browser.
You can also check the latest log file in the C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\LOGS\ folder. Look for the string "exception" near the end of the file. You can also look for "Telerik".
Are you upgrading from a previous editor version or is this a new install? Also, does the error still occur if you raise the application trust level to Full and restart IIS?
All the best,
the Telerik team