The preview of one particular material suddenly became dark, like shown in the first picture. The other material was fine.
Then teapots (from the scene) appeared in both materials 
I have uploaded the scene, just in case: https://db.tt/P3NGqLhx
looks like the “studio” objects are gone
I think it is Multiscatter that’s causing this. I was experimenting with Multiscatter and Corona proxies when I ran into this bug.
And speaking of Multiscatter, the collision feature doesn’t seem to work with Corona proxies (my guess is that Multiscatter treats these as objects that have zero volume).
ok, fixed