Players cannot join: mod mismatch

• "You cannot play/edit this mission" — the player is missing a mod, or has one the server does not have.
• "Session lost" straight after loading — usually a mod version mismatch after a Workshop update.
• "Bad version, server rejected connection" — the game build differs. One side has updated and the other has not.
• Kicked with no message — the mod keys are missing from the server keys folder.

Workshop mods update on their own for players but not always for the server at the same moment. After any mod update, restart the server so it pulls the same version your players just got.