Stian,
You're getting a different error than me so I can't directly comment. The only thing that worked for me was what was previously suggested in this thread which was to explicitly add both sites to the Intranet Zone in the Security Settings of Internet Explorer. I am doing this through Group Policy so that all machines on the domain get this setting.
As for getting more info from Internet Explorer you can try enabling script errors under Internet Options-->Advanced-->Browsing-->"Display a notification for every script error"