I was just searching for the same error and found this thread.
I just started playing this game again. Didn't play since alpha/beta so I started a new "solo" game without server just to play a bit. My brother also bought the game when he saw it.
He joined me with the steam overlay and disconnects after about 10 minutes. Even thought it was frustrating he kept closing and reopening the game to reconnect. That way he was able to reconnect all the time and play for 10 minutes everytime. Because he likes the game so much he did this 20 times with the same result everytime.
-Changing grafic setting didn't change anything
-He doesn't get an error and goes back to the main screen on which he can't reconnect "server unavailable" or something like that
-We are both playing on the latest steam version
-I also stay connected with a 0.5sec lag when he disconnects