Seems I'm having the same problem as Ringmaster.
Not only everytime I come back I have to log in again, but also browsing, it logoff randomly for no reason.

I thought it was cache but doesn't seem so.

Anyway:

Opera/9.51 (Windows NT 5.1; U; en)
Windows XP Sp3
forum Skin both Tigra and default, have the same problem

Cookies are enabled, I have the stoptazmo cookie.

I'll go back to use IE for a while to see if the problem persist