Attila - After thinking about your problem again I came to the conclusion
that it might be caused by flipped normals.While these are supposed
to render correct on a shaded object thea may cause problems with
reflections and so on.
Quote from the change log:
On surfaces that have inverted normals, diffuse lighting now renders as though the normals were not inverted. Flipped normals may still have consequences for other types of shading, such as reflection and refraction.
So maybe it's that.