Players cannot join your Project Zomboid server after a game update

Steam updates your players' games by itself. If your server is still on the old build, the players on the new one are refused until the server has the same build. Update the server with Steam Auto Update or Update now, or put the players on the branch your server runs.

Steps

1. Open Settings, then Game Settings. If a notice says A newer game build is out, the server is behind: click Restart and update now (Auto Update on) or Update now (Auto Update off). See How to update your Project Zomboid server.
2. Check Beta Branch. If it is a pinned or old branch, the server is not meant to follow the newest build. Either put your players on that branch, or change the branch: How to choose Build 41 or Build 42 for your Project Zomboid server.
3. Ask the players to pick the same branch in Steam: the game's Properties, then Betas.
4. If they still get file mismatch errors, they can verify the game files in Steam.

If the world is refused

If the server was moved to an older build than the world was saved on, the game refuses the world and stops: Your Project Zomboid server could not load its world.

Mods after an update

A game update can leave your mods behind until their authors catch up. If you run many mods, keep Steam Auto Update off and update when your mods are ready: Project Zomboid mods and Build 42: what to know.

Other join problems

Players cannot join your Project Zomboid server.