The “Registration Page can not be accessed directly” message is actually expected.
There’s a few moving parts to the issue you’re seeing, and one of them is the inclusion of the jQuery library from Google’s CDN. Ideally the included jQuery library should be bundled version that’s included with WordPress. It may be another plugin or the WP theme that’s switching the library.
Can you try temporarily switching to a default WP theme and temporarily deactivate all other plugins to help narrow down which one is loading the other library?