Hmm, that's really very odd. It looks like possibly that is the ground fog layer for some reason. If you turn off the ground fog in the settings, does that solve the issue? What OS are you on? Any error messages being logged in the RuntimeData folder inside your game folder?
If this is through Steam, you probably should verify the integrity of the local file cache and see if that helps. The usual other things like making sure your graphics card drivers are up to date, too.
I haven't ever seen this issue before, and it's super odd that it only affects part of the screen. Hope this helps!