QuoteATMO samples: 512
Cloud samples: 235
Render detail: 05
AA: 5
GI quality: 2
GI samples: 2
Ray tracing not enabled
Those settings seem to reasonable - except for the atmo-settings, they are really far off.
I stay for GI on zero, which cuts down rendertime enormously. In my last render with the spheres, I went with AA up to 19, to get really soft spheres, highlights and shadows. For this scenery an AA of 3 or 4 should be maximum.
Your clouds are quite thick. So you might want to reduce the atmospheric samples to 256 and go with the cloud samples to 300. Maybe that could be a clue.
Volker Harun