ok, retried and it installed.
but it's not launching.
opening the log shows:
Small bug: 1 unread topic listed, but "no unread topics" shows.
There seems to be some kind of mismatch here. I'm watching two categories (General and Development); there's a "1 unread topic" bubble, but when I go to https://community.nodebb.org/unread, I get a "no unread topics" message.
Probably a very small error somewhere, but worthy of correcting.
It's gone away on its own after posting this thread -- but it shouldn't have been in that state to begin with, right?