That was the point - thanks Elvince!
In my DBNetworkListener.h I had:
// Reference to the current handler
UDBNetworkBaseHandler* currentHandler;
and in DBNetworkListener.cpp:
UDBNetworkBaseHandler* currentHandler = handler;
I changed this to:
// Reference to the current handler
UPROPERTY()
UDBNetworkBaseHandler* currentHandler;
and
currentHandler = handler;
and it works.
So independent from all other checks and updates to the code it finally came to this litte word not placed? Unbelievable!
Thanks to everyone pointing me to the right direction. I will print this and pin it to my wall to never forget about it
So as a result in my opinion PIE is more forgiving than StandAlone or PackedGame. Nevertheless an detailed error description should have pointed that out earlier as mentiond by Elvince.