lightmapping problems in blend-model

Hey,

I've created this model in Blender. It's just a room corner. 2 walls and a floor. The walls are semi solid (front face, top face, back face, 0.04 thickness) and the floor is just a plane. I'm making the walls semi solid, because I want them to drop shadows on the floor face. I'm using 3 materials. Floor, Wall and a Clip Material for the outside. (I clip those faces away with a shader, to have no faces outside, but still get the shadow inside. This shader doesn't have any effect on that lightmapping bug)

Now I'm getting my Shaddow. But it's broken.

• http://imgur.com/a/qkwVl

• EDIT: close up: http://imgur.com/TNLUGCd

the second screenshot is from blender and shows a deformed version of the mesh, just to show that my edges are connected and that there are no holes.

Any Idea how to fix this?