Can’t edit “About” page!!

I go to edit my “about” page, and I get the following “Authentication required” error message:

A username and password are being requested by https://canadianenergyissues.com. The site says: “Magic”

I have no idea what to write in the error dialog: if I enter my login stuff it fails, and if I click Cancel I get a blank page which contains only the phrase “Access Denied”

Does anybody have any suggestions on what I have to do to fix this?

Thanks!