Short version: I have a world file that runs just fine until I add a single foundation, then reset the server. At that point, the server launches fine, but crashes immediately upon anyone joining.
Long version: Dedicated Linux server has run fine since launch, then crashed upon anyone joining suddenly after a building session. Extensive testing showed that placing a number of foundations consistently triggered the crash. After some trial and error, I managed to make a copy of the world where it was one foundation away from crashing.
This world launches fine. Can join, leave, close, re-launch, no issues. Then upon placing a single foundation and closing the server, it still launches fine, but as soon as anyone tries to join, it immediately crashes.
Notably, there are not an excessive number of foundations or buildings in the server.
Loading the broken save into single player DOES work just fine. Issue is only occurring on the dedicated server.