This in general seem to happen when the attempt at connecting times out: Basically there’s some kind of back and forth between the client and the server to validate the authenticity, list of mods, version of the game, etc… and if all that does not complete in some amount of time, then the whole thing fail.
Known issues have been servers running super slow (because of a weak machine or too many people playing on it, slow hard drive, etc…) network issues (on the server or client side), firewall problems, routing issues with the player connection going through crazy paths (can be checked with “trace route” tools), etc…
The easiest way to diagnose if it’s client or server side, is to try to get other people to connect to the serve...
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