We’ve tried everything to resolve our issue, including reinstalling the game, logging out and back into GOG, restarting the router, and opening ports, but nothing seems to work. We’ve attempted both invite-only mode and using the invite code, but when we select the other user and click “Invite,” the invite window simply resets and prompts us to make a selection again.

Any suggestions would be greatly appreciated!

One invites, the other joins or accepts?

What OS, praytell are you both on?

PC 1: Windows 10 - shows the Join by invite code, crashes when selecting that option (I'm guessing it expects to be installed with steam?), hosts a game, but doesn't accept connections over LAN anymore.

PC 2: Windows 10 - doesn't show the join by invite code at all.
PC 3: *buntu - doesn't show the join by invite code at all.

At the same time LAN Co-op stopped working (yes, the firewall is open on the host, UDP connectivity with netcat works fine on exactly the same ports.

In summary it looks like they added the join by invite code which broke the old way of running co-op and there's no explanation what criteria the game takes into account to even show the "join by invite code" option. There's a lot of questions on Reddit and Stardew valley forums, but no answers.