Hello, I’m using 2019.3.0f3 with URP and all of the asset thumbnails and previews for fbx and prefabs are fully black. The materials are fine. I have no idea what is causing this and it has been a while since I’ve been looking into this issue but it seems like nobody has it. I tried reimporting the whole project, played around with fbx import settings but nothing worked.
Something that it worked for me:
Select all the prefabs that you see black. Disable the Mesh Renderer from the inspector to all of them, and enable it again. That will force Unitys project file manager to refresh the preview of the prefab mesh.
I have the same issue after changing project to URP, nothing so far has been able to solve this.