The dark ring is probably caused by one of the sharpening pixel filters (Catmull-Rom or Mitchell-Netravali), as they can produce negative pixels in areas of strong contrast such as the edge of the sun. Switching to any of the other pixel filters should fix it. Cubic B-Spline is a safe choice.
Alternatively, if you enable "anti-aliasing bloom" you can use any pixel filter, as it blurs the sun edges enough to prevent negative pixels.