It happened that the central button of my click wheel was sunken by about 0.5mm . This is ugly as every time I selected something I touched as well the wheel and so the selection was wrong.
What was the problem: The center button and wheel sensors have a small plastic bump who presses the switch on the main board. To protect the switch they glued a black rubber pad ontop the electric switch. However, the one in the center was slipped away. So the button pressed direct on the switch. Thats why he was sunken in as the pad did not lift him up any more.
And her after the fix:
To fix it you have to open the iPod and adjust the rubber pad of the center switch.
Follow the following video to open your iPod or search for “iPod Gen 5 Repair Video” on Youtube.
http://www.youtube.com/watch?v=cqxuQQ9ERS0
Thanks to the one who created the movie.