Trying to start a new game session, I get "Failed to create a game session. Please try again later.", and trying to join a friend's session (he's on Windows) I get "Found no game session linked to this invite code.".

Is this a firewall issue?
In that case, what ports need to be open on either end?

I’m also curious about this, if anyone found a workaround.

EDIT: it’s because GOG Galaxy is required as with their previous games. Should be mentioned on the store page.

Thank you for pointing that out. Not because of Linux/Wine, but in general.
And yes, the need for Galaxy should be mentioned on the store page.

FFS... again!? This stupid fucking proprietary windows-only piece of shit galaxy software /o\

"Buy on GOG, DRM free!"
"Do i need your platform client to play?"
"Nooo, it's completely optional!"
"Cool! Bought it. Err, multiplayer no worky?"
"Oh yes, you need to use our platform client to play with friends."

What a shit time for gamers. Just let us enter the ip of the host machine - we're in local area network anyways! But nuh-uh, gotta force them players into one operating system and make them connect to our online services to make the product work (as long as _we_ let them, hahahahahahaha). And you know the best part? They still think it's DRM free! KEK

What even is the point of buying from GOG anymore? That we can "maybe" still use "some" features after the proprietary 3rd party online services have shut down?

So what now. Buy on Steam and "hope" they allow linux clients to download? Then run on the weak GPU since Steam has no multi-gpu support? Or buy on GOG and just accept not being able to coop?

Maybe should not buy the game at all and just skip it as yet another shit product that fails to deliver what they sold.