Project Zomboid Sandbox Settings says the file is from a different build

The Sandbox Settings editor reads Build 42 sandbox files only. If your server runs Build 41, or the file has been changed so that it is no longer one setting per line, the editor does not show it and says so. Edit the file under Files instead, with the server stopped.

The two messages

Edit the file by hand

1. Stop the server and make a backup: How to back up and restore your Project Zomboid world.
2. Open Files, then .cache, then Server. Click <Server Name>_SandboxVars.lua to open it in the editor.
3. Change the number or true/false you want. Keep each setting on its own line and do not touch the commas.
4. Click Save content, then start the server.

The file is a Lua table that the game runs, so a mistake, such as a missing comma or an unclosed quote, can stop the server at the start. If the Console then shows a Lua error, put the file back from a backup: How to restore only the settings or players of your Project Zomboid server.

Four values in the file are overwritten at every start: Zombies, ZombieRespawn, HoursForLootRespawn and LootItemRemovalList. Change them under Game Settings: How to change zombie population, respawn and loot respawn on your Project Zomboid server.

Use the editor again

Run Build 42 and the file is a Build 42 file, and the editor opens: How to choose Build 41 or Build 42 for your Project Zomboid server. If you restored a backup from an older build, the file may be old too.

The Server Settings editor does not check the build. It shows the settings that the file has: How to change Project Zomboid server settings in the panel.