Maybe it is a mod causing it, try this.
Open your config.php and below<?php add this line:
PHP Code:
define('DISABLE_HOOKS', true);
So it looks like this:
PHP Code:
<?php
define('DISABLE_HOOKS', true);
/*================================================= =====================*\
|| ################################################## ################## ||
|| # vBulletin 4.2.1
Then see if you get that error.