Sounds like this forum uses cookies for non-permanent logins aswell, in which case the 'problem' here would be that the time it takes for the cookie to elapse is too short (or for the people who know their php; that a cookie time has been set instead of omitting it which would make it behave more like a session).

Since I don't know the inner workings of this forum suite, can't help the moderators / admins with finding the right setting(s) in the control panel and/or help modifying the base code to fix the cookie problem.