Preview quality horrible after build (Grainy/Low res)

Hi guys, this is probably a very dumb question but you have to believe me when I say that I’ve been searching for an answer for at least a day now without results. Last night I added some foliage to my map and after building all of the sudden the quality was absolutely horrible (I’ve included some screenshots). Build quality is set to Max and Lightning quality to preview. This problem also occurs when I look at the sample first person map. Is there a Low-quality mode I accidentally switched on? I’m quite a newbie so please go easy on me.

Screenshot of what my preview dock looks like right now:

A bit more zoomed in: http://imgur.com/a/Y4HiQ

thanks in advance for the help!