Ok, have reviewed your install. YouTube is blocking the request. Specifically the HTTP call to YouTube responds with the following.

VIDEO_URL [status code] 429 [reason phrase] Too Many Requests

This typically means YouTube has blacklisted your domain. Typically this is just temporary. Will improve the error output of attachment in a future release and implement some additional fallbacks, but they won't be able to grab video titles and descriptions in those fallbacks.

It's a rate limiting feature at YouTube. Since we're making cURL calls it's going to rate limit your server and not necessarily your domain, sorry for the confusion. So for example if you're on shared hosting someone else could be slamming YouTube with cURL calls resulting in the server ip address being rate limited.

There may not be anything you can do if they've permanently blacklisted your servers ip address. I'm working on implementing some workarounds when the cURL call fails to allow trusted domains to fallback and still work as expected, but you will not have video title and description automatically parsed when that fallback has to be used.

YouTube is also being more aggressive with rate limits during COVID-19 outbreak so that is also likely a contributing factor.