Hi Olle, just to help clarify, the particular problem looks like an area of "self intersection" near the bottom of the blades. That's where some surfaces of the object crisscross back over each other, that kind of formation will prevent a solid from having a proper consistent inside and outside region and that can interfere with booleans and intersection calculations.
Here's the area in particular, notice the kind of curlycue crossing loop:
 Michael
