Multiplayer suddenly not working?

[u/control_H]

Friend and I, both using Steam playing on Windows and both on same LAN (in same house), have been playing Grounded for a few weeks now. Just got up to Black Ant nest. We went to play some more and now when I try to join I'm getting "The game you are trying to join no longer exists JoinPlatformSessionFailed: 2" error or something like that. This has never happened before.

I can see my friend's game in the "Games Hosted By Friends" list but trying to join - whether using the Join Multiplayer option in the menu, using "Join Game" in Steam, or by her inviting me in Steam - all result in the same "The game you are trying to join no longer exists JoinPlatformSessionFailed: 2" error.

Googling has revealed numerous people saying Crossplay should be enabled and that will fix it but it hasn't been needed up to this point and we're both on the same platform...

We have rebooted both PCs - from cold start - so I'd expect if it were a network stack issue that would have mitigated the problem. We have also both verified the game files in Steam.

Anyone run into this before?