There seems to be a conflict with Litespeed Cache and Youtube videos. My embedded Youtube videos can be seen if viewed from my laptop, but not on mobile devices.
I have checked with Site Origin Page Builder, which I am using, and they cannot find any issues with their plugin.
I have also reconfigured the Litespeed cache plugin according to the recommendations. But the issue remains.
But once I deactivate the Litespeed Cache plugin, the issue is solved and the YT videos show up.
I deactivaed the Litespeed cache plugin and tested it with another caching plugin, such as Hummingbird, and it has no such issue.