I have looked at my Cpanel and spoken with tech support. The default PHP running on the server is 8.1. The tech person lowered the PHP to 7.2 and the login page worked instantly. Their conclusion is that Sparkle is exporting PHP files that are not compatible with 8.1.
I can find nothing in the preferences or site settings which allows the user to select the PHP version upon export, though the documentation says “at least 7”. Is there a way to know exactly which version is being used? I think it would be best to know what Sparkle is doing, then match that on the server.
One problem / resolution I stumbled upon with the User Accounts. I was copy and pasting the email addresses from a couple different sources, and one of my test users just could not log in.
The UserName is an email address, but it also included 4 or 5 blank spaces after the .com, like .com______ (empty invisible spaces). I just happened to notice that my cursor did not line up after the M. Once I deleted those spaces, that person was able to sign in.