My Safari/Chrome computer is telling me that the Nabble HBBR site is not secure, written just before the page name, top of screen. Is this so? It hasn't happened with any other sites.
Firefox has been telling people the same thing for a year or more.
That message means that traffic between your computer and Nable's servers are passed unencrypted.
If you have not seen that message on other sites it either means that the sites you visit are secured with SSL or you only visit sites that do not require a password to view their content.
As all content on the UK-HBBR forum is on public display then it makes no sense at all to secure the traffic that travels between your computer and Nabble.
the trouble is that everyone seems to be increasingly concerned about privacy, so modern browsers have responded by warning users if the connection is not made through a "Secure Socket Layer" (SSL). You are probably aware of the technology, or the more modern TLS if you use any mail client rather than webmail.
SSL encrypts all traffic between you and the server of the web page. Look on the address bar of your browser and on a web page secured with SSL typically you will see a padlock symbol and a URL that begins with https:// rather than, these days, the frequently hidden http://.
While it is important that you don't pass bank account and credit card numbers and similar personal data over insecure connections, in my view, it is completely over the top to warn people about a lack of SSL on publicly accessible forums.