Hetzner, the server hosting provider, has informed me that they plan to do maintenance on the router that the Haven server sits behind. Not sure what level of redundancy they have around it, but I would assume the reason they're informing me of it is that the server will be inaccessible during that time.
For this reason, I'm going to bring the game server down during the maintenance window, so that the game doesn't become inaccessible while it's running. The announced maintenance window is 03:30-05:30 UTC on 2024-11-06.
Hetzner has a tracking page for the issue available here. Presumably, they will update it with the progress of the operation.