Found my leak today! Just in front of the hatch, where several layers of the pvc come together, left side, there it is. I inflated the boot and watered the top side carefully with a water hose, beginning at the hatch. Then I suddenly saw the water in the bow compartment. With a small mirror I looked inside the bilge and saw the water running down at the tube just under the front of the hatch.
Tried to contact my dealer, what do to - and was lucky: He had another one in stock and so I just switched them (made it just in time to the shop before they closed for the day; unfortunately the new boat is also still a 2008 modell, manufactured 2007). Looking forward to another 'peddle-trip!