Note: You are currently viewing documentation for Moodle 3.2. Up-to-date documentation for the latest stable version of Moodle is probably available here: error/hub/badport.
If you see this error when trying to register, it will probably be because you are using a non-standard port, eg (8888). Moodle blocks outgoing ports to reduce the potential impact of an outgoing attack in the event of a simple compromise (as well as other things.) As a result, only traffic to http (80) and https (443) is allowed.
If you wish to register, you need to run your server on port 80 for http or 443 for https.