What version of vBulletin is installed? What version of PHP and MySQL are on the server? Do you have suhosin or mod_security enabled (you will probably need to ask your host this question)?
Also, if only the admin account is having the problem, 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
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