As a service to other readers of this forum. The final solution was to purchase a support incident. The Plesk engineer replaced the corrupted auth.php3 file.
I did ask them ( and got no answer) if when I downloaded this version was the file already corrupted or was this something that happened during the installation process.
So if this happens to you, try re-downloading the 8.1.1 update, search through the cab files until you find this file and replace it, then let the readers know if that fixed it or not!