the upgrade from 4 to 401 should not effect login functionality.
the admin usename and password are stored in the Power Soter 4 database in the wa_settings table.
make sure you are using the same username and password stored in the wa_settings table.
if your still having an issue, test the servers session management using the session test page in the following post:
showpost.php?p=23826&postcount=2