Interactive video: 'Video format not supported' - mp4 in new modules, old ones still work

1. I created a video with mp4 format, uploaded to Vimeo Pro. Tried to embed it in a new H5P interactive video module; it doesn't display, a message in Step 2 (Add Interactions) says 'Video format not supported.'

2. Wordpress 5.3.2, PHP 7.3.

3. Desktop.

4. Same issue in Chrome and Safari.

5. H5P 1.15.0.

6. Interactive video editor, just updated it.

7. "A cookie associated with a cross-site resource at http://vimeo.com/ was set without the `SameSite` attribute. A future release of Chrome will only deliver cookies with cross-site requests if they are set with `SameSite=None` and `Secure`."

8. No PHP errors that I've seen.

10, 11. No recent changes to browser or environment other than routine plugin updates, but I haven't tried to create a new H5P video exercise in a few months, so hard to say.

Older H5P interactive videos on my site from last year, in the same format, still work fine in the browser. Embedding a new video in a new H5P module does not work; embedding an older video (that still works in an existing page) in a new H5P module does not work. 

Would be grateful for any suggestions!