If possible, check the server's event viewer. There should be an event in the Application log, which lists more details about the error. If you cannot find anything in the event viewer, you might be able to see the error details if you temporarily turn off the friendly error messages for the SharePoint site. To do this, open the site's web.config file and make the following changes:
1) set CallStack="true" in the SafeMode section.
2) set <customErrors mode="Off" /> in the system.web section.
Another approach is to use a HTTP proxy (e.g. Fiddler - http://www.fiddlertool.com/fiddler/
) to check the requests.
If you still only see the 403 error you can check the logs at \Program Files\Common Files\Microsoft Shared\web server extensions\12\LOGS\, which must contain information about the error.
Usually, the 403 error occurs if the user does not have read permission for the editor support files (located at \Program Files\Common Files\Microsoft Shared\web server extensions\wpresources\RadEditorSharePoint\18.104.22.168__1f131a624888eeed\RadControls\
). In your case the user should be NETWORK SERVICE, since you are using forms authentication. It is unlikely that the problem is caused by permissions, since you said that the editor loads fine. However, we recommend that you reset the permissions for the RadControls folder so that everyone has at least read permissions.
the Telerik team