I've been playing a co-op campaign with my friend with them as the hosts and we've been able to do it successfully for two game sessions. Today though both our gog galaxy clients updated, and now my friend seems unable to invite me. I also am unable to view any sort of multiplayer activity as the game says I'm not connected to the galaxy servers. Is anybody else experiencing this problem? Do we need to wait for either galaxy or the game to update before we can play again?

I'm also having this issue, and it's also preventing achievements from working - noticed after a few hours play that I didn't have some story progress achievements. =\ Oddly when I launch the game I see the pop-up for the overlay, and the cloud saves do sync up. I've tried every solution I can think of on my end, seems like a GOG issue.

I confirm. Achievements not working and no connection with Galaxy servers after the last Galaxy client update.

So I found something I hadn't tried, the GOG Galaxy beta client. And surprisingly it worked. As always with beta software though, your mileage may vary.