Disappearing thumbnails

Recently the thumbnails for one of my customers sites have switched to the default image.
Their site is http://soufflesandsawdust.com/.
The thumbnails were working fine. I’ve regenerated the thumbnails. I’ve tested by removing all other plugins and I know that your plugin works with this theme because I use it on several other sites with the same configuration.

What else could possibly be the problem?

https://ww.wp.xz.cn/plugins/yet-another-related-posts-plugin/