YouTube video not showing

Have you tried to embed the video?
https://codex.www.ads-software.com/Embeds

Thank you for your response, chienyi.
Yes, I tried. Normally it should show how the video would look like on the page, but now it only pastes as the URL.

Here you can see the difference between how embeding worked before (10 days ago) on the first picture, and how it works now on the second picture.

View post on imgur.com

You can try to inactive all the plugins and see how it goes.
If it works, you can add the plugins back one by one to see which one caused the problem.

I tried disabling all plugins, but the problem still persists.