Off the top of I my head, I don't know, I have not come across this issue.
Have you disabled all your mods and see if the issue still persists?
Open your
includes/config.php file and below
<?php add the following.
PHP:
define('DISABLE_HOOKS', true);
So it looks like this:
PHP:
<?php
define('DISABLE_HOOKS', true);
/*================================================= =====================*\
|| ################################################## ################## ||
|| # vBulletin 4.2.2
Use a editor like
notepad++ to edit any files,
don't use Notepad or Wordpad.
If that fixes the issue enable plugins again by adding
// before the line you added, then navigate to
ACP --> Plugins & Products --> Manage Products and disable one mod at a time until you find the one causing the issue.