@Ghandi: Could someone let me know why this answer is downvoted? Yeah, why? It has finally solved this problem ...
In my mind there is a bug when gradle or sdk or android studio is installed in a different directory then the standard one. I have S:\android\Android Studio and S:\android\sdk.
The solution by Ghandi installed gradle somewhere one more time, but I was really exasperated for this: one or two "gradle" more doesn't hurt me anymore