If the object has millions of polygons, Boolean calculations are heavy. Please be patient or try using a Decimate modifier on the object before slicing to reduce the polygon count.

You likely forgot to apply the scale. Select the original object, press Ctrl + A, and choose Scale. Then try again.

This happens if the object's wall is too thin or if the object is non-manifold (not solid). Try decreasing the Box Width to 4mm or 6mm to fit thin walls.