Most problems with an Astroneer server come down to a few causes. Start with the one that matches what you see.
The game cannot connect or is refused when joining the server
Why: The server runs with network encryption switched off, and a Windows client with encryption on is not accepted.
Fix: Close Astroneer, add [SystemSettings] and net.AllowEncryption=False to the end of Engine.ini in %LocalAppData%\Astro\Saved\Config\WindowsNoEditor\, save, and start the game again.
You joined but you are not the admin and cannot open the admin menu
Why: Owner Steam Name does not match your in-game profile name exactly, or Owner Steam ID 64 is not your own number.
Fix: Under Settings, then Game Settings, correct both values (the name is case-sensitive and must be spelled exactly), restart the server and rejoin with that account.
Players cannot join after an Astroneer update
Why: Everyone must run exactly the same game version as the server, and the server only updates when it starts.
Fix: Restart the server with Automatic Updates switched on and make sure all players have updated their game.
A change to the server name or password does not show up
Why: These values are written into the server's config file only when the server starts.
Fix: Change them under Settings, then Game Settings, and restart the server. Do not edit them by hand in the file, because the next start overwrites them.
The Owner Steam Name or Owner Steam ID 64 fields are empty and the server does not start
Why: Both fields are required, and the ID must be exactly 17 digits beginning with 7656119.
Fix: Fill in your exact profile name and your SteamID64 (look it up on steamid.io or steamdb.info/calculator), then start the server.
The console shows almost nothing while the server starts
Why: Astroneer prints very little to the console, even when it is running normally.
Fix: Wait a minute, then join with the Server Address from the Overview page. If you cannot join, open a ticket.
A player on Xbox, Microsoft Store or Switch cannot find or join the server
Why: Those versions of the game form a separate network from Steam and Epic.
Fix: Have those players join from the Steam or Epic version on a Windows PC.
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.
