Bug Project View Mini Thumbnails not generating

I'm working on a larger project and we recently upgraded to the 2020 LTS versions.
After that, we noticed that a bunch of our thumbnails for materials, prefabs, and meshes were not displaying.
I tried various things, deleting the library, redownloading the project, updating to an even newer unity version, but the only thing that happened is that they are all gone now. I even tried importing new models and creating new prefabs, but without success. In a new project, everything works fine.
I tried playing around with the AssetPreview class as well, but couldn't get it to display the correct icons.

The only thing I noticed so far is, that changing parameters on a material will generate and show the icon, but reimporting does nothing.

Help is much appreciated!

 

Bump!

 

Confirmed bug: https://issuetracker.unity3d.com/issues/preview-of-assets-do-not-show-in-the-project-window