This sounds like it's a problem with your browser's cookies, because that is what determines your login status to the forum. Clearing your browsers perefences, or changing the way it handles your cookies (and how long it maintains them) would be the culprit.
I use some other forums using the same software, but there the problem does not appear, so I think it's a problem of the Mellel-forum. But it is not really a problem, it's more kind of a cosmetic bug.