If you're seeing this on iOS, a quick fix is clearing the cache:
Head to Settings > Safari > Advanced > Website Data.
Search for youtube and swipe to delete.
Once you reload the page, the thumbnails should pull cleanly from YouTube's CDN.
PS
Talking about YT thumbnails I confess this one here makes me LOL ..
The coincidence was that I used the "Jenga Tower" analogy to describe when a software breaks .. the inspiration actually came from one of my favorites comic strips XKCD "dependency"
😁
and the whole point to use Eelco Dolstra's nix.
