Mojave: Finder can't load any thumbnails. Even after reinstall the system
Solution 1:
Sketch is making that issue. You can try it, delete sketch and all thumbnails will appear instantly.
I have reported a bug already.
Solution 2:
This issue has been resolved with the Sketch v52 update released today, as explained in its release notes (click View All and scroll down to the Bug Fixes section):
Fixed a bug where Quick Look previews weren’t generated when saving Sketch documents