Error: startup with Port while already running

[KaosKnite]

I've been playing a multiplayer game with my step son for a long time. Now we both get this same error when trying to create a MP session from a saved file. "Error: startup with Port while already running"
It's not a port forwarding issue with our routers because we can create MP sessions by starting a new game using same mods and map without this error showing up. This error only happens when we try to continue one of our saved games.
Has anyone encountered this problem and know how to fix it? Maybe changing something in the saved game XML? We have been playing the same game for a long time and don't want to start all over from scratch.