Hover a thumbnail at its bottom part and move the mouse horizontally to view the actual screenshots from the video
It works for me, so I can't investigate the problem. Youtube may be serving you a different version of the site as they often perform experiments. Note that the script works only if you don't enable the inline playback in youtube's options, because when it's enabled there's a built-in tooltip for the storyboard.
I figured out the problem, it was my VPN. Marking it Good.
Looks like they may have changed something again. Hover-over now displays an image that says "This content is unavailable" but the storyboard is still there, though the script may be getting pointed to the wrong image? Can you confirm and maybe fix @wOxxOm?