Sam commented on My Embedded App doesn't works in Rails 4.1

It looks like Rails 4 sets the X-Frame-Options header to SAMEORIGIN by default [1] which prevents the iframe from loading. Try adding the following to your config/application.rb:

config.action_dispatch.default_headers.delete('X-Frame-Options')

[1] - https://github.com/rails/rails/blob/4-0-stable/actionpack/lib/action_dispatch/railtie.rb#L21