Cura ignores whole in stl file

[haftting 0]

A friend created this object in Blender and as you can see it has a hole in the middle. When I slice it with Cura the object keeps the hole until layer 5 then it's getting filled which is strange. The original got extruded to add those little knobs/noses you can see and exactly at this layer (6) the hole gets filled up. Is there anything I'm missing here or able to fix this?

[ahoeben 1,968]

The model is probably not watertight/manifold. ie: there's likely a face missing somewhere. Check the model in X-Ray view.