Hi, i fixed it in 5.3.2 today, when i turned off “Forward Shading” and restarted Unreal. I had the problem in the last 5 days every time when i tried to create a new world. Go in the Project Settings and look for the word, perhaps it also works for you. 
For future people: can also try Alt+F or in the view-port in “Show” > Check “Fog” sometime your finger may slip and hit it on accident (happened to me).