Hi 3d2cnc, yes it's like Pilou writes above - you don't have a well formed solid there because the surface crosses over itself. A self intersection in the "skin" of a solid will tend to cause volume oriented calculations like booleans to fail.

How did you create the spiral part, can you post the curves used to construct it? Something like making it in 2 pieces that then get booleaned together may do the trick. Also if using sweep it would probably be good to use the "Twist : Flat" option so the seam edge would be in one location rather than twisting around.

- Michael