Apparently something changed in my configuration, and my linked post to Facebook yesterday changed the site url from http to https. This of course led to a warning when that link was followed, that the certificate was invalid (since I have no SSL configured), rather than opening the site directly. For various reasons, it is not possible to automatically forward to http, and I have no justification to add a SSL certificate (at an increased yearly cost).
Fortunately, the “fix” was fairly easy – just delete the connections to Facebook and re-create. After a simple test, this appears to have fixed the issue – my test post went in cleanly. However, it doesn’t really answer the root cause of the issue, so it likely will come back at some point (or even with this posting).
One more reason to abandon Facebook on my checklist.
