Hmm, not too sure on the logout issue. Is it happening to everyone, or just some? If it happens to someone consistently, then perhaps they can turn on the setting so they are always prompted whenever a cookie is modified. So as they browse around the forum, when they get logged out they should see the cookie being modified that contains the user id/session and then can post some details on what they were doing at that time.
On the errors you posted, it appears that someone is trying to visit the active topics list but isn't able to view any forums at all, resulting in the error. Which is a bit strange, since even when logged out, this works as normal.
