I assume that you have PHP 5.3.7+, as this version requires it.
Try following things:
- press Ctr-F5 to clear your browser's cache
- go to your /xoops_data/cache/xxx folders and delete the files inside them, except "index.php"
- go to phpMyAdmin and empty the "sessions" table
There might be an old session still stored somewhere. Let us know if it worked for you.