Try disabling your modifications/plugins and see if you still have this problem.
Note: To temporarily disable the plugin system, edit config.php and add this line right under <?php
PHP Code:
define('DISABLE_HOOKS', true);
If that doesn't fix it, then you should be posting over on vb.com for help with your default vB4 forum. See this sticky -
Installation Issues / Questions or bug reports regarding default vBulletin 4.0