When I try to update our PHP version from 7.4 to 8.0 I get a Critical Error message on the login page. The website seems to still work, but I can’t access the admin section.
Sorry to hear that. We have just checked the theme to PHP 8.0 and it’s working fine. May some plugin you have used are not compatible to PHP 8.0.
Also can you please share us the critical error message so that we can find out from where the error message is showing.
If you can not login to admin page just what you do is try renaming plugin folder to any name via ftp? and check.