[Lynne]

Run Maintenance > Diagnostics > Suspect File Versions. Do any *default* vbulletin files show up as Not Containing Expected Contents? If so, reupload all your default vbulletin files making sure they all get uploaded.

If you still have issues, try disabling your modifications/plugins and see if you still have this problem.
Note: To temporarily disable the plugin system, edit includes/config.php and add this line right under <?php

PHP Code:

define('DISABLE_HOOKS', true);