Generate Thumbnail functionality not working

The only thumbnail generator I can get to work is RoboThumbs. Most others simply display the closing slash bracket from tn html tag – like this />

I’ve tried signing up to PagePeeker and adding my API, but still, I get broken links. The image src generated is: https://api.pagepeeker.com/v2/thumbs.php?size=m&url=https://youtu.be/xxxxxxx. Oddly, no API key included here. Even if I omit the API key (which changes the URL to https://free.pagepeeker.com… I still get a broken link.

Any advice?