Nasa Target Log

The knowledge in this forum is my last resort....

I have a NASA Target log o/b.
When I start it up it shows 0.1 knots, then 0.0 knots. (so far so good)
If the boat moves through the water it always shows 0 knots.

I pulled the paddlewheel out and checked it.
It was ok, paddle moved freely like it should.
When I manually move the paddlewheel, correct readings come up the screen.

So, the system works like it should, but when the sensor is inserted, it won't work!

Also, i tired to clean the hole from the outside with a hose, a brush,....

The log is placed on a place with a normal, unobstructed, laminair flow.

Does anyone know the golden tip to help me?