YouTube video thumbnails not getting loaded On YouTube webpage, instead of the video thumbnail images, I see only a blue box with a question mark in it. Any video on YouTube does not seems to work either. Can anyone help me? I am not able to find a proper way to solve this error.

Last action performed before encountering the issue: Installing Unity 2018.3 (it is working fine)

I have a mid 2010 13" MacBook Pro running macOS High Sierra 10.13.6
I had a similar issue with the icons thumbnails that shows a blue box with a question mark in it. I'm running a Mac Pro mid-2010 running High Sierra. 

I finally got it to work after setting the DNS setting to Google DNS.
Apple > System Preferences > Network > Advanced > DNS Tab
And add Google's DNS servers
Click the plus (+) button below DNS Servers table and insert
8.8.8.8
and
8.8.4.4

Click okay and close Safari/Chrome and hopefully it'll be fixed.