It is indeed a very odd thing. π
I removed the length-snapping in the Units setting to see if that helped etc, but the box will rotate/snap into place, BUT not perfectly as there is always a ~3mm 'overlap'...
I suspect it's a tolerance thing.
The project is quite large, and very small and very larger geometry will always have issues...
Deleting the large surface below the stadium changes the overlap to only ~2mm !
You also have a couple of components ['Gruppe#nnn'] in the browser that seen to have no geometry in their thumbnail - perhaps their origin is miles from the geometry - which can cause weir clicking and snapping issues...
L