I am experiencing the same problem. It had been working okay.
I had noticed awhile back that two images were showing up side by side in category archives, Then in the last 24 hours, I started to get the message:
Error retrieving a thumbnail from the URL https://img.youtube.com/vi/nvjl90jY278/0.jpg
Libcurl error 0: Failed to connect to 2607:f8b0:4007:801::1004: Network is unreachable. If opening that URL in your web browser shows an image, the problem may be related to your web server and might be something your server administrator can solve.
There were several other videos on the page, and I found that if I deleted the first video (url), updated the post, Reset Video Thumbnail (which now displayed the second video on the post), put the first url back, updated the page again, Reset Video Thumbnail again, it would then pick up the thumbnail, but not set it as the featured post.
So I decided to try out a few other plugins. None of them worked as well, or as easily as this one, so I reactivated this one but it was still not setting the thumbnails as featured images. I deleted all of the other plugins I had tried (in fact I disabled all of my other plugins), and optimized the db, but to no avail.
Used with Thumbnail For Excerpts, this plugin has been a nice addition to WordPress for quite a while. So sad…