I've seen this when "Save server state between runs" is enabled, and the server is restarted while users are connected. This was soon after the feature was first introduced, and we haven't had it turned on out side of testing since so it may have been fixed by now. It looked as if all the session data was being given to the wrong session ID when the server started up again. I don't know if this is the same problem you're having or not, just thought I'd toss it out.