I have a solution:
Rendering without ground and using the occlusion shadow.
Exporting the render with clown mask
Separate the object using the clown mask.
Rendering with shadow into the background.
Transparency of the background to for example 70%.
So I found a solution for my problem. Here is the workflow. Creating rendering with an environment with a background color of 50/50/50.
Also aground plane with a shadow color of 50/50/50.
Render it out with a clown mask.
Import it in a program like Affinity Photo, Photoshop.
Duplicate the rendering.
Import also in there the clown layer.
Mark at th clown layer the outer area.
Hide the clown.
Mark the render layer in the front.
Delete the shadow by using the eraser rubber thing.
Mark the background render layer. Change the transparency.
With this workflow there are no white pixel in the shadow which don't fit to another color than a white background.
I did a few renderings (.png & alpha) for customer with occlusion ground shadow. He wants to use the renderings now in a power point with half of it with an orange field. He doesn't like the shadow white in the outer areas.
He doesn't want me to place the orange on the ground in KS. He wants to be flexibel. I tried also without occlusion shadow. But then the shadow goes too far outside and is cut off in the Power point.