Okay, based on the screen shot, IIS is giving it's default 400 error path for the request (/error_path/400.html)
The first request on your screenshot (Default.aspx) is the call to the CMSMOdules/Content/CMSDesk/Default.aspx, THIS is the ajax postback for the response in question and since it shows 303, this may be what's wrong (Normally shows 200)
If you can provide it's Headers and Response here, that should help us further! It may be returning an error that we can use to get closer to the solution.