I came across the following solution to a similar problem:
1. In SE38, execute the report WDG_MAINTAIN_UR_MIMES. Under "Menu:, double-click "Delpoy MIMES". Then when that has finished, double-click "Force MIME deployment".
2. In SMICM, choose the "Goto --> HTTP Plugin --> Server Cache --> Invalidate Globally. Click "Yes".
3. Delete Temporary Internet Files of Internet Explorer: Tools--> Internet Options --> (General Tab, Browsing History section) Delete.
Worth a try :)
Adi