@bryan Thanks a lot for your reply. Unfortunately thats not the solution. Even if i use a larger image (you´ll see it on the demo-page) – the problem still exist.
The problem may be that the media-query for tablets are set to 1024px. The new tablets have higher pixels, so that the media-query of the desktop is taken. Thus the tablets use again background-attachment: “fixed” and not “scroll”. Then the distortion arises.