Fritsje on "Youtube embed not working anymore after 4.5.1 update"

On my blog I have about 800 posts/pages, many of them have a Youtube-movie on it.
The Youtube link is simply pasted into the editor and then even in the editor a preview can be seen, and on the published post or page the video can be watched.

Since the recent WP update however, all the videos are not displayed anymore but only the Youtube URL, like https://youtu.be/xx1SAjrqkIo

So links are now on my posts and pages, and not movies.

On all my other sites nothing has changed, all videos are working just fine.

This problem is browser independent, I have disabled all plugins and still the problem exitst. I have switched to WP 2014 theme but also on that theme it does not display the videos.

I did not use a plugin for displaying Youtube videos. The URL look the same in both WYSIWYG-editor and TEXT-editor.

Repasting the url in the editor does not resolve the issue, neither does enableding or disabling "WordPress should correct invalidly nested XHTML automatically" in Settings->writing.

Anyone has any idea what causes this or how to fix it?