Share 'pixelated fragments in finished render?!'

I got some weird fragments below my finihsed rendering somehow. do you recognize what could be the reason and how to get rid of it? I used a brazil volumetric environment the first time! The presettings are high AA, default bounces, indirect illumination, skylight on, and quality slower and smoother. The materials are glass and a kind of brushed aluminium for the plane. Should I increase the bou…