A mi me paso lo mismo y lo que me comentaron en soporte fue esto y si resulto.
Try adding this line to your "includes/config.php" file to see if the white 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);
Or your forum style might be corrupt. Try reuploading the original "install/vbulletin-style.xml" file and then running "tools.php" (in the do_not_upload folder of the zip file) to reimport the style.
All the best,
Jake Bunce
Support Team, vBulletin
