Probably another of Youtube's little tricks. This seems to happen on some, but not all, new clips.

When I try to play the clip, the frames are all there but the video won't actually play. (Won't move.)

But it does play after I manually advance the player to about 5 seconds from the end, then back to the start. After this forward-and-back, the clip plays normally.

Seems to be a 'signal' somewhere near the end that tells the player to start moving.

When I tried to use Gom Video Converter to fix them, Gom said the file was corrupted.