That, kind of, was the problem, thank you!
Here is what happened: I try to learn from a Marketplace-project. But instead of copying everything, I try to adapt it to my concept. The cast that always failed was in the Marketplace-project, after another level was loaded. I, however, did not load this level and thought, just changing widgets would do the trick. As it turns out, the other level has a different default GameMode and, thus, the cast did not work for me.
Now, I load another level with suitable default GameMode, PlayerController and so on and everything works just fine.
Thanks again!