This locks like it is a training data problem. If training data locks like this, it will generate pictures accordingly. Because the picture is nicely clean, it is maybe the better way to use a editing tool and clean out the background manually.
Next, NEVER mention something you don’t want in the picture. Negations not work, use only positive descriptions from what you want in the image.
It can help to repeat what you want multiple times, and see if this works. for example “the background is clean and purely white”.
here is al link with some help-tips
(German? I make the same mistakes sometime 
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