Quick answer: ECO connection issues: port 3000/3001 UDP, matching mod versions and server fully started (2–5 min).
If you landed here: ECO shows "Connection failed", server not in list or mod mismatch on join.
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Frequently asked questions (FAQ)
Which ports for ECO server?
3000 UDP (game) and 3001 UDP (web/API). Open both in firewall and panel.
ECO mod version mismatch?
All mods on server and client must match version. Check mods folder on server.
ECO server not in list?
Server needs 2–5 min to start. Slg1 world corrupt? Load backup. Check server name in config.