Your Terraria server stops because the world is from a newer version

An older Terraria server cannot open a world saved by a newer Terraria, and it stops with a crash instead of saying why. This happens when the server is pinned to an older version, or automatic updates are off, and the world was played on a newer game. Set Terraria version back to latest, switch Auto-update server on, and restart.

What you see

On Terraria Vanilla, before the server starts, the Console shows a warning from the panel:

[pnh-update] WARNING the world <name>.wld was saved by a newer Terraria than this server's build (<build>), and Terraria cannot open it: it will stop with a crash. Set the Terraria version to latest and restart, or choose another world name.

Then the server stops while loading the world. The crash can mention NullReferenceException.

Fix it

1. Open Settings, Game Settings.
2. Set Terraria version to latest.
3. Click Show advanced (1) and switch Auto-update server on.
4. Restart. The server updates to the newest build and loads the world.

Or, to stay on the old version, load a world that version made: change World Name to another world. See How to switch between worlds on your Terraria server.

On TShock and tModLoader

• TShock: after a Terraria update, TShock can take a few days to release its build. Until then it will not open a world the newest Terraria saved. Wait for TShock, then restart with TShock Version on latest.
• tModLoader: it still runs Terraria 1.4.4, so it cannot open any world saved by Terraria 1.4.5: tModLoader cannot open your Terraria world.

Good to know

• A world opened by a newer Terraria is saved in the new format and cannot go back to an older version. Keep a backup from before an update if you might need the old version.
• More: How to update your Terraria server and hold a version.