Can't login after upgrade
 

Long story short, my host upgraded our server which rendered my VBulletin useless, nobody could login at all today. After several hours they came back to me and said VBulletin says we need to upgrade, so I upgraded.

Still can't login, even the admin panel login page generates this error:

"The requested URL /forums/login.php was not found"

Where did we go wrong in the upgrade?