I'm using CS 16.2.11 with a Chrome browser
If I place the Welcome Header widget on a perspective of mine, the thumbnail displays and the video plays if I press play. But if I try to add my own video & thumbnail, the thumbnail won't display and the video won't play, no matter what the URL I enter. It doesn't matter whether the video is in CS or if I point to a YouTube video.
When I press play, I see this:
And If I click the play button at the lower left, I see an hourglass for about a second, and then nothing else happens. The timeline stays at 0:00.