Two player share a home modem/router network and internet connection, and play DayZ together. We like to play together on the same servers, but sometimes experience trouble doing this.

The problem occurs when one player joins the server, then the other attempts to join them. The second player cannot join the server, and receives a 'unknown error' popup and the game returns to main screen. They cannot connect to the same server at the same time. The first player can continue without any problems.

A solution that we used to use to fix this problem, which does not seem to work anymore, is as follows: we used a command line option in the DayZ properties section of Steam - right click DayZ, general, set launch options, then e.g. -port=25009.

Both of us used a slightly differnet port number. This method used to work, but it stopped working around the time when the DayZ standalone launcher was released on Steam. I noticed that the standalone launcher changed the way in which we could make entries into the launch parameters, so I wonder, is it possible that this 'port' setting was accidently disabled or its implementation modified during this transition to the launcher?

A solution we have recently found is to reset the modem-router before attempting to play. Once the internet connection is re-established, both players can join the server and play together. This is a reliable solution but pretty annoying. I mention it here because it might help to understand what the problem is exactly. We have a totally standard router with no custom settings implemented.