I spoke to early. The problem came back.
Then I decided to wipe everything obsidian from the machine, reinstall, resync and start over.
This is windows – I did the uninstall, reinstalled and was a bit surprised that I was automatically logged in after the reinstall. (and still had the problem). I uninstalled again and discovered that there were still obsidian folders in “appdata.” This time I manually deleted those files and reinstalled again.
Then, as expected, I needed to login again. After syncing, this time it seems to be working. So now I suspect there was something corrupted in the appdata folders.
Of course I have no idea what was corrupted, but I think the most actionable suggestion is that an uninstall should delete those folders.