Most problems with The Forest server come down to a few causes. Start with the one that matches what you see.
The server does not show up in Steam's Add a Server list
Why: The game port was entered instead of the query port.
Fix: Remove the entry and add it again with the server's query port (shown on the Ports & Network page), then click Refresh.
The world starts fresh after every restart, or your old world was replaced
Why: Starting World is set to New, which generates a new world and replaces the old one.
Fix: Stop the server, set Starting World to Continue and start it again. If the old world was already replaced, restore the backup you made; without a backup it cannot be recovered.
One player cannot join while others can
Why: Their game is on a different version from the server, usually after an update that their Steam has not applied yet.
Fix: Ask them to restart Steam so it updates The Forest, and keep Auto-Update on for the server. If an update breaks connections for everyone, turn Auto-Update off to stay on your current version.
The server runs but is not in the Steam server list
Why: No Steam Account Token is set, so the server does not register itself with Steam.
Fix: Create a game-server login token at https://steamcommunity.com/dev/managegameservers and enter it as the Steam Account Token, then restart. Otherwise players can still join with Add a Server.
Some players are refused when they try to join
Why: Enable VAC is on and those players have a VAC ban on record.
Fix: Turn Enable VAC off and restart the server.
Anyone could become admin on a new server
Why: A new server starts with a shared placeholder admin password until you change it.
Fix: Set your own Admin Password before inviting anyone, then restart the server.
A password is rejected when you save it
Why: Passwords may only use letters, numbers, dashes and underscores. The Server Password allows up to 20 characters and the Admin Password needs 6 to 20.
Fix: Pick a password that fits, with no spaces or other symbols.
Still stuck?
Open a ticket in the category that fits and include the last 50 to 100 lines of the console. See How to open a support ticket and How to fix a server that will not start.
