Why do I get a "This webpage was blocked by an extension" error

The error "This web page was blocked by an extension" occurs when trying to access Gmail from a Chromebook.

This issue is typically caused by an out of date revision of Chrome OS. Please update the Chrome OS.

https://support.google.com/chromebook/answer/177889?hl=en

If you Powerwash and Reset your Chromebook, one of the first things that it does is update the Chrome OS for the latest release.