Well, after setting shortcut in Xcode to CMD+SHIFT+7 it ended up as CMD+SHIFT+/ but on German keyboard the key / does not exist without modifier key, but in fact this is a different shortcut.
So for me the solution was quite simple in the end: using the numbers of the NumPad to assign a shortcut in Xcode works …
CMD+SHIFT+NumPad7
Hope this helps.