Thumbnails are not displayed

Hello,
I really love your plugin. Unfortunately, I have an issue for some days. I just switched from single pictures to wordpress galleries (no plugin) in most of my posts. Now, there are mostly no thumbnails being displayed in the Related Posts section. It worked before. If you want to have a look at it you find my page at https://www.bangforthebug.com.
Thanks!
Andreas

https://www.ads-software.com/plugins/yuzo-related-post/