When I go to the Account settings and click 'Update' on either my billing or shipping address I get an Error 404.
May need fixing
404 Error is page not found. It means those page which you have open does not available on the website.
I pointed it out because I can't update my address, which means the website may need fixing.
Just checked. Still hasn't been fixed.
First off all delete those page which does not open and show the 404 error and after all fix the error 404 in the google webmasters.