It looks like the gog and steam versions are out of sync: attempts at cross-play fail with error "Server returned an error: Code 'Version'"

It's highly probable that the reason is that the steam got an update a couple of weeks ago.

Any ideas on when/if/whether the patch might be available on gog as well?

The plans for this evening to have a blast in Vran's coop went down the drain :(

I have same problem. And worse yet, there was update Sept 09th which added local coop multiplayer support. I am really looking forward playing that.

When will GOG.com version be upgraded to this version?

I am also waiting for the update. I have a friend on Steam that I can't play with because the game versions are not the same. I'm pretty disappointed it takes GoG so long to get things updated...

Good news, everyone,Gog has just released the patch!

I haven't tried it yet myself, but it should fix the things for now (well, at least until the next update comes out).