Your selector is way too a-specific.
class = combobox can refer to any combobox in your application, even hidden ones.
Try making the selector more specific, or anchor it to a visible label in the window it belongs.
Also, in modern design experience:
revalidate your window selector
it can even help to target the popup as a new window, so using a separate (not nested) use application/browser container.
I have also tried to open a new application/browser container to include the window “Umfangbearbeitung” but also here, the selector does not work properly
I has 2 same windows with the same name opened at the same time. I ensured that one is closed and after that UiPath could recognize the anchors and the target.