I have a similar square-ish problem, only not so intense. The small magellanic cloud has visible edges around it for me. (I'm using the 0.9.7.3 version)
Look closely to the top and bottom of the cloud. If you zoom the image in, you can clearly see the border, as if it was cut off. If you don't see it, then you may want to increase the brightness of your monitor or something, because it's clearly there.