After what I thought at first: Mist, the aperture vignetting yes at small focal lengths, I then encountered in the DSLR forum on the cause and the solution.
I just quote again:
"I guess rather that you (also by JJC) have not properly set up until it engages the lens hood. This is namely a common operator errors. Many users shy away from the necessary effort in order to turn the initially stiff Streuchtblende until it stops, as they afraid to break something or can not imagine that you still can continue to turn. "
Just as it was. The first time's needs right force, then it's then easier.
Since then, no more vignetting. Everything as it should be - TOP !!!