YouTube videos are not working when Elementor and Slider Revolution is installed

The root cause of the problem is a YouTube API conflict.

Explanation

Revolution slider always uses the https://www.youtube-nocookie.com domain to display their videos, which means we have to use the https://www.youtube-nocookie.com domain as well, regardless of the YouTube And Vimeo Privacy Enhanced Mode option. (So even if you disabled the option, it will act as enabled to be able to stay compatible with Revolution Slider.) Otherwise our videos would not work, as YouTube does not allow using both https://www.youtube-nocookie.com and https://youtube.com/ on the same page to embed videos.

At Slider Revolution you can't change the YouTube privacy mode. At Smart Slider, it can be done at the Global Settings → General → YouTube And Vimeo Privacy Enhanced Mode option, which will make all videos created by Smart Slider use one URL. However, at Elementor you can change the Privacy mode setting of the video at every single video widget. As a result, if you don't enable the Privacy mode option when Slider Revolution is enabled, or when you turn on the YouTube And Vimeo Privacy Enhanced Mode option of Smart Slider, one of the videos will not start

Solution

Make sure you turn on the Privacy mode at your Elementor video widgets if Slider Revolution is installed on your site, or if you enabled the YouTube And Vimeo Privacy Enhanced Mode at Smart Slider.