Thanks for taking your time to reply.
It is running now after I did what you said. After ReturnBoolean an CustomEvent is called via CastToGameState.
I am a bit confused now as I thought any BP could do what GameState does in this case. Guess I need to read it up again.
For others with this issue:
You can’t link GetGameState() directly into BindTo(). It needs a CastToGamestate.