I've had the same issue when playing with a friend. We've traced the problem to being with the amount of civilizations when joining a game as well, specifically it can handle 15 but not 16.
This is not an issue when you're actually in the game, it's only when trying to rejoin.
Sadly this has forced us to play without Revolution and BarbarianCiv in an effort to control the amount of civilizations in the game at any one point which subtracts from the overall game value.
As far as network logs go, we're seeing the same as Saew. However, the other party is also getting a socket error - "The network socket encountered an error and was closed" when he is hosting the game, something I am not when I am hosting the game (it still doesnt' work though). This also occurs if we try to use PitBoss to host it.
The error from network-2056.txt when running with PitBoss, hosted by my friend:
I have errr... misplaced my own network-2056.txt file but can get one if needed of course. However, what is worth noting is the line
[105036.750] WRN: Our connection to NetID 1 was closed remotely
This occurs on both machines - of course it would be NetID 0 in my log if he was hosting. Again, I receive no socket errors neither by popups or in the logs, and neither does he if I host the game.
I'm really at a loss as to what the cause of this is, I have only been able to identify the syndrome as being caused by a number of civilizations greater than 15.