Once again I have to bring this up. There seems to be another problem which is not caused by the atmosphere but looks quite similar.
Image one shows the mentioned problem with objects casting shadows onto the atmosphere with a reduced opacity or a greyscale image.
Just to describe the settings again: There's a visible, not shadow casting ring object with the glass shader transparency falloff method for the diffuse color. Then there's an identical ring object (slightly moved downwards to not intersect with the other one), which is invisible and shadow casting. I just used a default shader with a greyscale map for the opacity.
In image two "receive shadows from surfaces" is off. The ugly bright area is gone, but the red arrows show some weird terrain artifacts where the terminator and the shadows intersect.
The third image shows the same problem with the atmosphere turned off, and the artifacts are still there, so it's not only caused by the atmosphere.
Image four shows the planet without rings, just to see, how it should look. No problems here.
I have no idea what the problem is here. It's just a semi transparent object casting shadows on another object. There is no problem on a smaller scale.
While I'm writing this, I just had the idea, that the location of the planet (far, far away...) could be causing this (for whatever reason), but I made a test with the planet at 0,0,0, and it was still there (see last image).