If you previously built from source, make sure you uninstall that first. For whatever reason, even though the opam version correctly reports "1.1.0", it was still using the old repo address. After you uninstall old opam entirely, then follow the installation instructions at the link above.
You will know you succeeded when you do "opam update" and prints:
default Downloading http://opam.ocamlpro.com/urls.txt [NOTE] The repository 'default' will be *permanently* redirected to https://opam.ocaml.org (opam-version >= "1.1.0")