Hi MateuszW93,
Glad map situation isn’t occurring for you anymore. Please let us know if problem starts happening again.
I have not been able to confirm whether change to make multiple game instances on same machine unable to connect was intentional, so I’ve entered a bug report for it (UE-24894). It appears this was fine in 4.10.0, so if absolutely necessary to test on one machine, you should be able to grab 4.10.0 version from GitHub and build it from source. Otherwise, testing on separate machines should work fine.