Strange Problem with Blender 2.40

The image below has been rendered with Blender 2.40 and Yafray. As you can see, the image is convered by several black spots. The problem occurs when I use subsurf on the box and no XML is generated in Yafray settings, if I unactivate subsurf or I generate XML in Yafray settings, the problem does not occur anymore. Any idea ? Thanks.

Are you using a Spot Lamp with RayShadows?


No, one sun light (no shadow) and one area light with sample=2.
I also use an HDRI image as light source.