6
Find a fresh copy of the XOOPS version you are running. Download it from a totally different mirror than the one you downloaded from before and download it again.
Open the compressed XOOPS and get the system folder from the modules folder and upload the system folder to your server overriding the one that is there.
Try logging in again and accessing the admin.
P.S. I sure hope you have backed up your site and database before trying any fixes.
Quest