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 criss-cross 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 curly-cue crossing loop:





- Michael