I suspect a PHP error of some kind. Try adding this line to your "includes/config.php" file to see if the page shows any specific errors:

ini_set('display_errors', true);

Does it show any error messages now?

If that didn't help then try adding this line to the same file to disable plugins:

define('DISABLE_HOOKS', true);

If that didn't fix it then try reuploading the original PHP files from the vBulletin zip file.

It might also be a memory limit problem. Try adding this to the config file:

ini_set('memory_limit', -1);

All the best,
Jake Bunce
Support Team, vBulletin