Make a video search; repeat until you see a result with a missing thumbnail.
Solution:
Sometimes /maxresdefault.jpg doesn't return a real thumbnail.
A fix is to switch it to /mqdefault.jpg or /default.jpg on line 53 of engines/invidious/video.php, although the resulting thumbnail will be at a lower quality.
Example:
https://search.ahwx.org/search.php?q=Italy+losing+the+Euro+Cup+2024&p=0&t=2&nf=1
Reproduce:
Solution: Sometimes
/maxresdefault.jpgdoesn't return a real thumbnail. A fix is to switch it to/mqdefault.jpgor/default.jpgon line 53 ofengines/invidious/video.php, although the resulting thumbnail will be at a lower quality.