Hi Uwe,
The object "COLLINETTE" used as a Surface has flipped faces (thought in the view-port it doesn't seem so - please feel free to do some additional testing with the xView - Face Orientation tool or a 2Sided material). From the scene itself it's not clear how that issue arose, but in similar cases Flip normals > Reset XForm - Reset Selected > Convert to Editable Poly fix it. Attaching the modified scene.
Best regadrs,