This board is overrun with BOTS, the "Session" table is full continuously not allowing any new sessions (if you have a cookie for the site you can still get on)...
I run a vBulletin forum and ran into the same problem last month, I increased my max table size from 16M to 64M but I also added the following code to my hourly and half hourly cleanup files to delete "guest" sessions (i.e. the BOT sessions), haven't had a problem since...
Code:
$vbulletin->db->query_write("
DELETE FROM " . TABLE_PREFIX . "session
WHERE lastactivity < " . intval(TIMENOW - 600) . " AND userid = 0
");
HINT to the site moderators... You are getting a bunch of vBulletin Database Errors because your session table is full, adding the above code to your "includes/cron/cleanup.php" and "includes/cron/cleanup2.php" will solve your problem...