I'm not quite sure what has happened, but the 1940 US Census for Scott Township, Marion County Ohio, ED 51-49 when posted on Ancestry.com is totally unreadable. The pages ARE readable on the images on the National Archives site.
The only things I can guess are:
1) Ancestry turned UP the contrast on the ED. Since the printing was light to begin with, and Ancestry has been known to turn up the contrast in the mistaken idea that brighter is better, this could have happened.
2) There is a serious issue with the beta reader - although I have NOT had an issue looking at any other Township.
If someone from Ancestry is monitoring this board, this needs to be fixed. The issue is on your end, not the census side of the equation.