I noticed Apache was mis-configured and was serving liblfds pages from other virtual domains on the server.
I fixed it.
This broke my configuration. (I hadn’t really fixed it.)
Apache is hard to configure because the docs are all over the place and there are a dozen ways of doing the same thing and there are plenty of strange behaviours built into the server.
I’ve backed out HTTPS for now, to get things working till I have time to sort them out properly.
(As an aside, WordPress is a bit crap. If the site URL changes, you can only fix it by editing the mySQL database directly – it’s not actually config in a user-editable text file…)