[eluser]WanWizard[/eluser]
Using cookie-based sessions?
If so, then there's your problem, a cookie can only hold 4Kb of data, which you reach very quickly due to the encryption overhead. If not, is the database column 'user_data' large enough?
If the cookie or the payload gets corrupted due to truncation, the session is no longer valid and a new session will be created.
