I believe ubuntu 20.04 should be set to 7.4, or possibly 7.3, definitely set all your default php versions (and update paths of default php in server config). I don't know that it'll fix your issue, as I don't know what's causing it, but give it a try.
Thanks, Jesse! It looks like php-cgi never got updated - it may have been removed in the upgrade and never put back. Once I figured out that "php-cli" and "php-cgi" weren't the same package, I realized that "php-cgi" didn't appear to be installed. I did an "apt install php-cgi", and that installed it and updated the alternatives. When I restarted Apache, the ISP Config login prompt came up. I think we're good now. I really appreciate everyone's help!
Not to beat a dead horse, but why, then, was that the on;y PHP package that wasn't at 7.4? Before I installed it, I used dpkg to list the installed php packages, and all of them had a 7.4 installation except this one.
