This is the precalc of the "max quality" attempt, and I think we can see where this is going - there are two lights in the scene, both set to cast raytraced shadows of 5 degree blur, min shadow bias .1.  I still think this is coming from insufficient stochastic samples, although I am not by any means a render engine gearhead.  This link leads to some pretty old info but the concepts are still valid (old version of Cinema 4D).

http://mvpny.com/RadTutMV/RadiosityTut3MV.html

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