Sure I have writed 2 for 1 :D C'est ok maintenant :)

For the problem
1 you have 2 boxes :) Click Move the box :there is another box inside! :)
But that was not the problem !

2 Move all the "boxes " out of the "object" Kill one of the "box"
Select only all the "pied a coulisse" : make boolean Union
Put the "box" at the good place
Make Bollean Diff
All works as a charm :)