Hi, yesterday I player a multiplayer game on local network, it worked fine. Today, using the same computers, when I try to join the game (entering the server's IP), I get an error "private server". What does this mean? It worked fine yesterday and doesn't work today with the exact same configuration. Thanks.

Note : I have disabled firewalls, each machine can see the other on the local network.

Check your multiplayer settings in the Game section of the options. Alternately, you can check in-game from the Connection Menu (diamond icon, top of mini-map).

Right, I actually found out a few minutes after posting... I need to enable local multiplayer in the game settings. So there is a bug there, because I was able to create a multi game and have another local PC joint the game even though none of the clients had the options enabled, and it worked! Then second time it didn't work without enabling the options.

If you start a multiplayer game, that automatically enables LAN visibility for that session (Direct Connections will be visible if that is set in the options). When you restart and load a save, it will use the options you have set for both (which default to offline).