I am also having this issue starting this morning. When the Adguard extension for Firefox is enabled. I am unable to log into my Gmail account, and unable to play Youtube videos. When I enter my username and proceed to the next step of entering my password for my Gmail, I get the following.
For Youtube, when I click to play there is just a circle spinning. I have also noticed the little Adguard symbol on the top right of the screen where it shows how many things it has blocked. The number keeps going up and up until it gets past 100, then it has the infinite symbol. The only way of being able to login to Gmail and play Youtube is disabling adguard.
I also since today like 2 hrs ago ( i guess there was an update) my youtube videos stop loading, after searching for the problem everything got fixed when i disabled this extension.
Also using windows 7 x64, Chrome version 83.0.4103.116