random database errors

[ATVTorture]

I'm getting database errors for a reason unknown to me. Most commonly, they're generated by one particular user on our forum, but occassioanlly from guests.

It seems most common that the referrer is showthread.php if that makes a difference.

These are the latest ones:

Database error in vBulletin 3.7.3:

Invalid SQL:
SELECT IF(visible = 2, 1, 0) AS isdeleted,

NOT ISNULL(subscribethread.subscribethreadid) AS issubscribed, emailupdate, folderid,
thread.*

FROM thread AS thread

LEFT JOIN subscribethread AS subscribethread ON (subscribethread.threadid = thread.threadid AND subscribethread.userid = AND subscribethread.canview = 1)

WHERE thread.threadid = 3354;

MySQL Error : You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'AND subscribethread.canview = 1)

WHERE thread.threadid = 3354' at line 10
Error Number : 1064
Request Date : Tuesday, November 11th 2008 @ 08:42:59 AM
Error Date : Tuesday, November 11th 2008 @ 08:42:59 AM
Script : http://mysite.com/forums/newreply.ph...uote=1&p=16436
Referrer : http://mysite.com/forums/showthread.php?t=3354

--------------- Added [DATE]1226419658[/DATE] at [TIME]1226419658[/TIME] ---------------

When trying to view this user's profile, I get this:

Database error in vBulletin 3.7.3:

Invalid SQL:

SELECT COUNT(*) AS count
FROM user
WHERE referrerid =
AND usergroupid NOT IN (3,4);

MySQL Error : You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'AND usergroupid NOT IN (3,4)' at line 4
Error Number : 1064
Request Date : Tuesday, November 11th 2008 @ 11:04:34 AM
Error Date : Tuesday, November 11th 2008 @ 11:04:35 AM
Script : http://mysite.com/forums/member.php?u=569
Referrer :

I've also noticed if I make a change to the user in user manager and hit save, it says it saved the change. When I go back to the account, the change did not save.

We've created a new account and merged this one into it so they would lose posts and PMs. Account worked fine for a short while, then blew up again.

Any ideas?

Thanks!