Likely actual issues with internet connection or Steam. You can make the game wait for response for longer by using the "/timeout" command.
Thanks man. It hasn't solved the problem, but has helped to narrow down the cause. Now we know it is not caused by connection, but rather by the game freezing. It just stops and it doesn't let you Alt+F4 it, so you have to finish the task to close it. After 30 seconds - 1 minute (maybe changing the timeout value makes it longer, so maybe it freezes until we timeout. Not sure tho) the game comes back to life, but we are disconnected from our game and have to continue alone.
It's super annoying, as it happens very often and ruins most of the runs. Sometimes it even happens when we pause the game, so we don't think it is related to the amount of stuff that is happening on screen.
Any ideas?
The game freezes if there's nothing coming through from the remote player(s), meaning that it is unable to proceed without dropping connection to player(s). Timeout regulates how long the game waits this way before giving up.
Amount of things going on has very little significance due to technical aspects - since the number of objects gets very high late-game, networking is done closer to how RTS games usually work - transmitting actions instead of positions.
My suggestion was to try setting the timeout to a very high value (e.g. 3600 = a hour), and seeing if it eventually comes back or not. The technical side is as simple as it gets (asks Steam to send data back and forth; pings twice a second if nothing comes back), so it is unlikely that anything can be done on the side of the mod; Restarting Steam sometimes fixes particularly strange issues with it.