it looks like a sampling error. Try upping the samples, also turn reflectiions off so that you can elimate that as the problem. Also I'm not 100% on your occlusion render. Shouldn't there be occlusion on the ground below the object too?
Try increasing the number of rays etc to up the samples (i'm no MR expert though! )
Technical Director - Framestore
Currently working on: Your Highness