I give up too.
And after rebooting everything - all computers on network, router, modem - I did manage to get it running fine on one of my laptops, using 8787 (8080 still not working). Not ideal, because I don't usually keep calibre open on that one, and I do keep it open on another machine almost all the time, so metadata.db can't sync regularly between them. But it'll do well enough - I'm just glad to have gotten it working at all!
Thanks for all your help!