Multiplayer co-op matchmaking/session connection fails if any participating player does not have their Windows system clock configured to sync automatically (Set time automatically enabled). When a host or client has manual time synchronization active, connection handshakes fail, preventing players from joining or hosting sessions.
Steps to Reproduce:
On Player A’s (or Player B’s) PC, open Windows Settings > Time & language > Date & time.
Toggle “Set time automatically” to Off and manually adjust the system time by a few minutes or hours.
Launch Subnautica 2.
Player A hosts a co-op session.
Player B attempts to join Player A’s session via invite or lobby code.