Im with ct mate....a planar face does not need thickness to boolean into....the two 'surfaces' are what gets booleaned. I think adding thickness will then create 2 booleans as it will try to interpolate 3 surfaces instead of 2. Can I ask why the box normals are reversed??? There are 3 options for booleans and they will operate differently according to which surface you have picked first....give us a screenshot that will help.

cheers bullet