Hey Rhino team,
This new strange thing has been happening since yesterday. When I scroll through my material library, everything at first looks normal. But as soon as the preview is rendered, all the thumbnails turn super dark, like there’s no light in the scene. But that’s totally new. I have been working on this file for a while. So I thought, maybe it’s an issue only with that file. But no. When I open a fresh new file and create a custom material, it also turns super dark as soon as its preview is rendered. So I checked the render settings. And checked if Metal is still my render engine. And I just can’t find a solution to this problem.
I have no clue if I accidentally changed a setting, and I also searched the forum for possible solutions. But for example, changing the Gamma back to 2.2 did not change anything. Only the rendered viewport looks different then. I am a little lost, to be honest. It’s just kinda tricky to work with a material library in which someone just turned off the lights. 
Maybe this video will help to explain my issue.
Thanks a lot for your help.
Kind regards,
Tom