Judging from the last image you posted and the capture of your settings, I'd recommend changing your "Optics" settings. An fstop of 39.6 is fairly uncommon (most lens that aren't specialty top out around f22), and adjust your shutter speed to something more realistic. Shutter speed in your settings is set to 1/.02, which isn't something you'd ever see in the real world (again, Maxwell relies on realistic settings for its light calcs). Maybe start with 1/30 and go from there?
It doesn't look like the file is getting errors, and your sky isn't quite "black" as you say. The image is just severely underexposed. Hope that helps! Also as a quick note, for interiors, Maxwell will render significantly cleaner and faster is the interior isn't completely sealed from the outside environment. Transparent materials don't contribute, so try to have one portion of the model open to the outside, perhaps, behind the camera or somewhere out of sight.
