warum wurde dieses foto aus qualitätsgründen abgelehnt

Ask yourself what buyers will use this image for commercially.

Also, which keywords will they search to find it? 

Then compare your work with current Stock inventory to see what you’ll be competing with for views & sales. 

Hope that helps.

I agree with ​@yamato713108855 that exposure is an issue and the shadows could be opened up a bit. I wonder, too, if the buildings on the right might be cropped out.

The white building in the background appears slightly overexposed (blown-out highlights).
Images with blown-out highlights or crushed blacks (loss of detail in shadows) will be rejected.

Images containing text or logos—such as those found on street signs—will be rejected due to intellectual property issues.
Please edit the image (e.g., using Photoshop) to remove them before submission.

If people's faces are visible, a model release is required.
Since obtaining permission from each individual may not be feasible in this situation, it might be best to remove the people using Photoshop.
Alternatively, you would need to aim for a time or moment when no people are present in the shot.