my test site http://u2.hol.es/
I have checked it on your site. It seems only Youtube thumbnail is affected.
I think you have "Set default YouTube video thumbnail quality" to MaxRes. It's disappear because maxres thumbnail is not available for every YT clip. Please set the config to "high" (stands for hqdefault.jpeg)
The maxres settings does work with 4K videos: https://www.youtube.com/watch?v=iNJdPyoqt8U
Regards,
Thao