I am having the same problem, although mine is a new installation, not an upgrade. When any users try to log in they get a blank page. The URL usually had a sid number but sometimes it just has "login.php". Also sometimes it will give me a page not found and sometimes just a blank page. Before I was able to hit back and be logged in, but now I just can't log in at all. Before when I was able to log in by hitting back, I could not log out. I have the boards on a yahoo webhosting server (I know, I know, i wasn't my descision.
) so I'm thinking maybe they don't support all of the PHP functions? I tried deleting those cookie but it didn't change anything.