Blender 2.8 Cycles All Materials Same Color As Viewport

I don't remember changing any settings that I don't know what are for. The color of all materials is exactly the same as the one of the viewport background, ignoring roughness etc. This also occurs in final render. Eevee works fine, problem is only when using Cycles.

What have I done wrong? Any solutions?

• I had the same issue, but because of something different. I did not realize I had Pause Preview enabled. Now I know that option exists too. – Ruben Sanders Sep 2 '19 at 19:51