After much grumbling and groaning, downloading Dropbox again fixed everything!!!
posting in case this helps anyone
Yay!
For anyone else reading, the very first thing to check with any sync issue is that Dropbox itself is working on all involved devices. A Mac OS update a few months back silently disabled it, for example, so even if it was working last week, check again.
If Dropbox isn’t working, nothing else you try will help. Moreover, we (Scrivener support) can’t fix Dropbox issues.
(Easiest way to check Dropbox? Create a test file in your local Dropbox folder, then use your browser to check your account at dropbox.com and see if it appears.)
Katherine