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.