Missing geometries in a 2D plot group

Hi,

My modell consists of a magnet between two ferrous pole shoes. In the air gap between those pole shoes is a ferrous triangle. I'm using the live-link interface to import this geometries from Solidworks into Comsol.
Additionally there is one air-box with a fine mesh surrounding the triangle and the air-gap and one air-box surrounding the whole setup.
I am trying to visualize the magnetic field lines on a cut plane through the air gap of the pole shoes. The picture in the attachment shows a white triangle, where the field lines should be.
Obviously there are lines going inside the triangle and leaving it on the other side.
But why isn't Comsol able to show the white region of the plot?

(I assume that this error message maybe could be linked with my problem: If I want to make a parameterized surface of an air-box which contains no iron I get this message:
Cannot evaluate expression.
-Plot:previewplot(surface))

I was spending hours to solve this problem and now im really frustrated. I hope someone can help me.
Thanks in advance!
Julian