This was exactly the point, thanks! I found a solution here: Arduino Forum
BTW: well, why should I? It's very simple. I asked for a hint (and not for the entire solution) and wrote I'm beginner. So if somebody is friendly to me, I'm friendly too and vise versa. How do you say that in english, tit for tat? Asking people whether they are clueless idiots shows enough about the character of such people. Anyway, it's not a problem for me because I don't care about such people.