This error appears, because you are starting the game im the Editor. When starting a Game with 2 or more players in the Editor, rather than starting 2 or more copies of the game, the players will be directly connected. Most of the time, the viewport player is the server. If you use a new game window, you can see who the server is at the top bar of the window.
The problem now is, if you host a game now, the connection they already got will be cut, because the server is starting a new server and the old one will be closed.
So this is quite normal when testing it in the Editor.
The thing with the not working server hosting is strange.
This could be due to not loading maps or destroyed widgets.
Are you sure that the server starts a map? You might want to create a new GameMode for the other map and check in the outliner if it is changed. Otherwise a picture of the Host logic would be nice.
I know the complete Shootout Game, so i won’t be that confusedm (: just put in some pictures.