What browser, and what settings? This doesn't happen here, so there has got to be some kind of disagreement between the site and your browser. If we can get some of the particulars, the browser and the site can be forced to hug and make up.
Aha! I think I found a problem! When I did the last engine update, the installer renamed the MZ cookie to the default engine cookie name. This would for sure log you out if you visit another site. I've just renamed the cookie, so that problem should be gone now... Sorry to anyone online at this exact instance, because you just got logged out
It seems, that the problem is back. I just logged in, replied to a entry, and got the msg you are not logged in. I relogged. Wrote the same again. And was just logged out.