Joining IP fails

I have two different levels:

  • MainMenu
  • Map

The MainMenu has a gamemode override for the GameMode default (in other words basic mouse over UI only works)
The Map has all the custom gamemode with character pawns etc that I created for my game. Basically they kick in the moment the map is loaded.

My hosting script found in the MainMenu is:

The Host Game works perfectly and the game starts with the map and expected game modes. (all the print strings are displayed in both clients but this is another bug to figure out)

The Join Game when clicked seems to be creating some stuff on the editor (world outliner) but then flashes back to the main menu. It seems that the Map is never loaded.

Why is this happening?