I've got a similar issue. The newest Linux download on GOG (stardew_valley_1_5_928320980_43631.sh ) reports to be version 1.5. The newest Windows download on GOG (setup_stardew_valley_1_5_928320980_(43630).exe) reports to be 1.5.1. If I host a session with the newest Linux version, I cannot join the session from Windows because of the version mismatch.