Nieminen Mika wrote:
>   You can see from the image, that two of the normals are pointing to one
> side of the triangle while the third is pointing to the other side. This may
> be causing the problem. I think that all the normals have to be pointing to
> the same side of the triangle.

This, in fact, seems to be, or was, the problem. That triangle was 
the only one,  of thousands, where the normals were reversed from 
one corner of the triangle to another, and it was the only triangle 
POV was not accepting. This is easy enough to detect and work around.

Thank you very much for your efforts and insights.

Now, my question is why does POV-Ray do that? What is wrong with
one normal pointing 'up' while two point 'down' (or vice versa)?

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