Yesterday I experienced quite annoying "Couldn't load plugin" error when I tried to play any video on YouTube. It started suddenly and I was very surprised by it.
As a quick fix I enabled YouTube HTML5 Video Player but it didn't worked for all videos due to advertisements.
After a little digging I found dirty way to solve this problem that actually works:
$ rm -rf ~/.config/google-chrome/PepperFlash
To know more about this problem read Chromium issue 173790: Flash player component update to 126.96.36.199 fails on Linux.