Hmm check your htaccess file:
https://www.vbulletin.com/forum/foru...-now-down-hard
i.e. rename it to .htaccess1 temporarily, then try the upgrade script (rename back when done). This *might also be related to PHP version you can try switching/swapping that out if you have an easy option in cPanel such as "Select PHP Version" but just note the modules installed on the current (print screen button/paste into paint etc).