Most certainly memory related issues. First thing I noticed was the AA of 10. That really isn't needed for this scene since you don't have any objects at all. Try changing that to around 4-5. Also you have a lot of black empty space, simply crop that out and then re-add it in post, that will get your overall dimensions down somewhat. If you made it to 80% changing these two things should get you the rest of the way. If not, then split the image into two crops and that should work fine as well.