First, see if it is a plugin/product problem by 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);
If that doesn't fix it, then here is a page from the manual about troubleshooting blank or 'white' pages:
https://www.vbulletin.com/docs/html/blank_pages