It is due to https://commons.wikimedia.org/w/index.php?title=TimedText:Editing_basics_-_Uploading_and_adding_images.webm.en.&action=history

which is an incorrect subtitle for that video, which makes the player logic trip. Deleting that page should fix it.

The broken subtitle file has been deleted, and I can confirm the video now plays fine, at all places it was before failing.

Before this is closed, I would like to highlight how this is still broken, in a sense, in two ways.

First the way this problem is reported to users is terrible. A user or editor on experiencing it might think the media is broken, or their browser does not support that format (both common occurrences at times). It’s only taken so long as the root cause is so non-obvious. Compare it e.g. to script errors, which generate red error messages + call stacks.

Second this makes it potentially an attractive vector for disruptive editing or vandalism, as there is no validation or checking of these files. This broken file was created by a disruptive editor Craft6789, Their other edits were all reverted but no-one thought to check and remove this file until this problem was noticed, a year later.