Why does this STL not slice properly?

[Mjolinor]

Learn how to fix it or you will most likely run into the same issue again, and you will be right round in a circle. If you don't know how you caused it when you modelled, then chances are you most likely will repeat the problem. Learn from your mistakes

I can't fix it, I don't know why it happened. I have seen Freecad do it before on much simpler things where you just have a couple of things intersecting sometimes it just makes a real cock up of doing the subtraction.

Time to learn Blender methinks.

[Roxy]

I don't know what you did to create the design... But when I run into this problem it is usually because I put two things so their edges are at the same location, but they don't intersect. Suppose I want to stick two 1x1x1 cubes together. If I put one at the origin and one at (1,0,0) I am going to have one face of each cube in the same location, but not actually intersecting. This has cause me problems again and again. If I put one at the origin and one at (.9999,0,0) everything will be fine.