Great idea, but unfortunately, that still doesn't solve it. I am beginning to think, that it doesn't have anything to do with the loading itself, but instead another script isn't happy with loading the game. It could also be a bug inside Godot, but I am not so sure about that. It is just weird that this occurs, since it worked fine before.
But anyway, thanks for the answer, i will play around a bit and see if I can finde the problem.