2 Friends and I played a fair amount during Early Access and would on occasion some times have a problem connecting to our Co-op game, but usually after a few restarts of clients it would usually get there.
Yesterday evening we booted up the ‘released’ version and initially could not connect or ‘Join Game’ from the steam menu, as Server not Found, or Not Playing on a Dedicated(?) Server or something.
However after restarting again it allowed us to join and play fine on our original Game/server.
Today the brother of one of our friends bought the game and played for 5 hours with said Friend on the co-op server. They then logged out and had lunch, and since then the ‘brother’ cannot connect to our co-op game, or any other co-op game we create. He can launch his own co-op game or join an Official Server, and we can’t Join his Co-op game???
We haven’t had to do anything with Port Forwarding or such before, AND he played fine for 5 hours this morning???
Please can someone suggest what may be going wrong?
Thanks in advance 
From the Steam Forums I found out that a lot of people are having similar problems.
Once you or your friend have created their New Game, log out and use Continue Game.
Hopefully you will be able to Join Game now.
However we are experiencing random Unable to Join’s from time to time, endlessly Unable to Connect to Server, similar to Early Access, but usually after a few restarts of both Clients it eventually connects.
It feels like some kind of synching or connection error somewhere, but I have no idea why it happens some times and not others.