The error is related with the Hosting Server PHP Version. So if you are on PHP 7.3.XX try downgrading to 7.2.xx or 7.1.xx and see if that helps!
P.S. I was facing the same issue and had to downgrade to PHP 7.1 to fix it!
Forget to mention: I tried to comment out muted in #99 of plugin file as suggested here: https://html5backgroundvideos.com/background-video-addon-siteorigin-page-builder/#div-comment-612 but that didn’t help.
