I had and got rid of it... I remember boosting the photon count to get rid of the blocks. But maybe it was the interpolation. Or just plain luck. Or all those other corners I looked in. It happens mostly with planes isn't it? The parts where no shadow interactions i,s are sometimes colored slightly lighter. So maybe it also has to do with the quality settings of the lights you use, when using the soft shadow option. The default is 'fast', which sounds not too good. I hate it when I solve a problem by changing too much parameters at a time, so I can't remember which did the magic thing. Or which combination.