It still looks very much like the Shadow Min Bias error, as suggested by hborre earlier in the thread.  Exactly the same error occurs on almost any model when it's set too low.  

Whatever the value is on your Diffuse light, increase it.  Try doubling it and see if that works.  If so, reduce the amount until you find the lowest value you can use without the error appearing.

Maybe the culprit is a combination of that and two sided polys. 

Coppula eam se non posit acceptera jocularum.

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