@bearer Thanks for the response.
I was all set to say "yep, the same", but I figured the least I could do is set it to another IP address in config.d and go there.
So. Many. Hours. Wasted.
Looks like something else grabbed 192.168.1.77 while my printer was offline and wouldn't let it go, causing a conflict. I just set Duet to 192.168.1.222 and the web interface came up without issue.
Bleh. Much appreciated!