and another problerm with a different setup.
finaly i was able to get the smooth result but i noticed again the
problem with dark areas in refelctions. for some reason refelctive
objkects doi not get reflected with their refelction. they are black.
Could you post the blend files for the last two images where you are having black patches in the reflections?
Judging by the image alone, I would say there must be something wrong with one of your materials. I say this because, in the image with the cornell box and the 3 balls and that curved object, you can see a ball reflected in another ball, but the curved object appears black. The curved object even reflects itself at some point and there too it appears black. Strange.