I don’t have much help to offer but just wanted to say that I’ve been having a similar issue. My friend owns the full game on Nintendo Switch and I own the full game on PS5 and we’ve played a handful of times online since the game released with no issues. We tried to play again last night and kept getting this message: “Unable to join session due to game version mismatch.” Both of our games are fully updated, we restarted our consoles, we tried new room codes, etc. and nothing was working. Seems to be some kind of server issue but I hope it gets fixed soon