I really don't know. I'm not that familiar with this server error. You can read this -
http://www.checkupdown.com/status/E400.html I'd try turning off all your plugins and seeing if that fixes it.
Note: To temporarily disable the plugin system, edit config.php and add this line right under <?php
PHP Code:
define('DISABLE_HOOKS', true);
Don't forget that for vbseo, disabling the modification means to also get rid of anything extra you added to your htaccess file.