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Your ground plane is simply too large. At most, I would use something 10x as big as your space. Yours is 4000 times bigger. This causes the interreflection calculation to reuse the cached ambient value from the previous point when you give it all your points at once. In any case, the results will not be accurate, unless you turn caching off with "-aa 0".

I recommend you use a smaller ground plane (and obstruction) and specify a background below it by extending your sky to cover a 360° angle rather than 180. This is standard practice, but perhaps not as well-documented as it should be. Also, I would double-check the options you are using with gensky to make sure they are the ones you want. I don't think your -R and -b options are used consistently, and you may think you are specifying photometric units when they expect radiometric ones.