OK ... I give!!!
I am trying to upgrade a working 5.02 to 5.31 stand alone PP, and everything seems to be good, except that I cannot login at all, neither as a user or as admin. The login process simply takes me back to the main page logged out, even though it states that login was successful.
I have cleared all cookies, cache and temp files, and nothing seems to work, whether I use IE or Firefox. I've re-uploaded all files, I've gone back to 5.02 twice, and upgraded again, only to get the same results each time. I'm now at a complete loss.