404 When Adding WHMCS URL


I can't get this set up. When I add the remote URL for the WHMCS installation in the WordPress plugin settings, upon clicking Save I get a 404. And the URL is not saved.

EDIT: It seems to save (and not throw a 404) when I enter the URL without http:// or https:// (which, of course, is not a true URL).

This brings up another concern: Is the communication between the WordPress installation and WHMCS secure?