Check your session table in the database, you can login cpanel then go to phpmyadmin to check the table in the your forums database. If the session table has around 30,000+ listed then its an issue there.
IF and I mean IF the session table has an insane amount of entries (like I mentioned above around 30k) then it could be your timeout setting i.e. how long the members can stay logged in - If you set this too high without modifying certain files it can cause an issue. IF the session table is full then run this query:
PHP Code:
TRUNCATE TABLE session
http://www.vbulletin.com/forum/forum...-table-is-full
*All my comments above should only be read and any directions followed IF you checked your session table first and IF its full or around 30k entries.