Bingo! This problem is solved. The underlying issue was poor connectivity through a tunnel providing a single ip address associated with the Discourse forum. While tests via other sites including external ones had not shown this, your recommended test via mobile (which I should have thought of, but with all the other tests I’d done I just didn’t think of it this time) did show the performance problem, which apparently was bad enough to prevent certificate renewals from completing (via the route that Let’s Encrypt was using). This was straightforward to solve once the tunnel was indicated as the culprit, and a rebuild of the Discourse app immediately pulled in renewed certs, so back to full operation. Thanks Jonathan and all! Have a great upcoming New Year!