#Getting a strange error, still in version 3.7, could be the cause.
4 messages · Page 1 of 1 (latest)
You do need 4.0 to play on any current GC server, but also the server you're trying to play on is offline.
As for updating, download either 3.7->4.0 hdiff (requires you to use an hdiff updater) or download full 4.0.x client
As linked in the setup guide both in the pinned thread and on the front page of the grasscutter github