@Gargron you're right, I hadn't noticed the two stes of <html>...</html> before!
There isn't any indication of compromise in my system and no stray index.html (the whole mastodon direrctory was blown away then live was pulled in from git again). But there is progress. I *can* go to https://.../web and it will let me in again, but that initial redirect from the root URL still does the poco thing. Maybe something is cached somewhere from the first botched upgrade?
@msh Eh, could be, then delete the nginx cache directory and see what happens