Keep Getting This Error

[Mastar]

I keep getting this error ever so often:

Database error in vBulletin 3.5.3:

Invalid SQL:

   SELECT userid, username, email, languageid FROM user
   WHERE usergroupid IN ()
   AND inactivitytag=0
   AND lastpost=0
AND pmtotal <= 0
AND lastactivity < UNIX_TIMESTAMP(DATE_ADD(NOW(), INTERVAL -180 DAY));

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 inactivitytag=0
   AND lastpost=0
AND p
Error Number : 1064
Date         : Tuesday, February 14th 2006 @ 01:39:12 AM
Script       : http://www.mywebsite.com/cron.php?&rand=444803
Referrer     : http://mywebsite.com/index.php?sid=fe31118cc7cc96e26557f3d6ac47dc8c
IP Address   : 66.150.200.169
Username     : Unregistered
Classname    : vb_database

And this one also:

Database error in vBulletin 3.5.3:

Invalid SQL:
;

MySQL Error  : Query was empty
Error Number : 1065
Date         : Tuesday, February 14th 2006 @ 02:11:45 AM
Script       : http://www.mywebsite.com/
Referrer     : http://www.mywebsite.com/login.php
IP Address   : 23.166.98.132
Username     : gghh
Classname    : vb_database

[amykhar]

And the last mod you installed was?