Hi Ussnorway, thanks for your reply!
We are all located in Germany, round about 60 km radius.
Anyone knows how the game / Larian establish the connection between the clients?
It's peer to peer, the person starting the game/lobby is the host. So connection issues is usually due to local reasons, for which could be many causes. Though fixed IP is unlikely (very unlikely) a fix.
I'd recommend emailing supportbg3@larian.com on this