I doubt that this is a DNS issue. Since I’m traveling right now, I don’t hand the time to look into it more. Perhaps someone else can.
My best bet is that YouTube is preventing embedding its content. How and where the messages originate from remains to be seen.
I suppose that dragging and dropping the URL simply creates a link. That’s something YouTube can’t stop you doing. And copy pasting it in a MD or RTF file in DT should work, imo. If it does not, we’ll need more details.