I found migrating from joomla3 to joomla 4 that the URL to an article containing a video works only if that URL links directly to a Menu -> Article. In case the URL has a link to a Menu- Categorylist - Article I get a page not found.

Editing the article of a category-list in the preview mode the video works properly.

Any idea what is missing?

Solved.
I apparently missed out a Joomla 4 setting for the category-list