Blender actually does NOT delete material connection
and when I select the wall and add a Red New material
you see that it is being attached to the Mirror geometry!
Even when I delete all materials and material index
save and restart Blender some materials are remaining
in the file and being somewhat attached without showing
up as being attached in the material buttons!!!
I also imported the objects into new Blend files but
the problem persists.
This makes Yafaray crash all the time because the material
information is corrupt.
The only way to get Yafaray Rendering again is to actually
export the geometry as OBJ and than import.
Appending carries over the problem what ever it might be.
I also appended just geomtery with no material and it crashes as well.