I don't know if the issue has been resolved in any way, but I am now getting much the same message if I click on an eMail link from the address no-reply@canalworld.net advising that a member has posted on a topic I am following. I got in now via my Firefox history which showed a connection I made a month ago:
Did Not Connect: Potential Security Issue
Firefox detected a potential security threat and did not continue to url7653.canalworld.net because this web site requires a secure connection.
What can you do about it?
url7653.canalworld.net has a security policy called HTTP Strict Transport Security (HSTS), which means that Firefox can only connect to it securely. You can’t add an exception to visit this site.
The issue is most likely with the web site, and there is nothing you can do to resolve it. You can notify the web site’s administrator about the problem.