Spline making a huge terrain hole

Hello,

My own curbstone is causing trouble. It looks like this in Blender:

and I've made sure to remove doubles and reset both rotation and location. The grey lines are the separate cutting mesh.

But when placing it in the map editor (z=0.0), it creates this parallelogram-shaped hole:

In the CT, the cutting mesh is set to "only cutting, the mesh stays invisible" and with alpha=2.

The visible meshes are set to "No cut".

But why is it then still misbehaving? Even though I've done pretty much everything after the book/lexicon?

-Thor

Thank you Florian, it works perfect now.

Yet another point to add to my personal checklist

LG