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Based upon the tests I've run so far, I see no evident reason why the challenge file would be unreachable.
If you put a file named test inside the /.well-known/acme-challenge/ directory under your webroot directory for billing.matttechbackups.com, are you able to successfully access the contents of that test file by visiting http://billing.matttechbackups.com/.well-known/acme-challenge/test in a browser? Please be sure that you're checking if you can access the file using HTTP in the URL and not HTTPS as Let's Encrypt will first attempt validation over HTTP on port 80 then follow the redirect to use HTTPS on port 443. If there is a problem with a rogue rewrite/redirect, this test path will help surface it.
I'm starting to wonder if you might be right about CertSage being unable to create the challenge file. I find this difficult to believe though as I built specific error checking into CertSage for that very situation. Perhaps I should add a secondary existence check after each directory and file creation. Adding that to my next release . Clearly LiteSpeed is able to serve files from that directory. What are the permissions on these directories:
(webroot directory of billing.matttechbackups.com)
(webroot directory of billing.matttechbackups.com)/.well-known
(webroot directory of billing.matttechbackups.com)/.well-known/acme-challenge