Thanks, I already have a very solid idea why it happens for me: Because I have Referrer-Policy: same-origin set. This means no Referer for embedded Youtube, breaking the embed. A similar effect could occur with strict tracking protection settings in browsers.
That’s fine though. I mean, I would certainly prefer it if the Youtube Onebox would use youtube-nocookie.com, which may or may not help, but that’s a different topic.
Instead, I want to understand and fix Lazy Videos. It’s superior anyway. But I have no idea what to try.