How exciting. ;) I have had this issue on occasion but I don't think I've ever solved it the same way twice. Sometimes adding a backdrop (even just putting in black as a color) has fixed it. Other times I've changed the render resolution from the default 72 to 144 or higher. Most of my stuff is rendered at 300ppi and I never see the problem - I thinks it's just at the lower resolutions that it happens more frequently. For whatever my instincts are worth, I'd try the backdrop thing first. The "rings" aren't occuring in places where the light has interacted with something.