This is from the dendron.install.md file, you can search for "youtube" and see the same image, except the for the file: dendron.topic.publishing.quickstart.md. Comparing them I think I see what is going on. The "working" example is an inline html link to its video with a still image to be shown before playing, the other youtube videos are specified via markdown with no such still image specified. Whatever MPE is using as its rendering-browser is not capable of playing video, and as these videos have no still image, this pops up! I expect if you took away the still image from any of the other video links, you'd get this "apparent" error too.