Nothing is wrong with the certificate, as it does not exist.
This site does not use an SSL certificate.http://www.gwtwforum.com
If the site used a certificate you would be connecting with https, not http.
You probably are getting a "non-secure" warning because the the site does not use SSL/TLS. More and more browsers and systems are pointing out non-https sites these days. Could be your browser or OS was updated recently and this is why the security notice is new.
As there is no need for credit cards and such on this site, no specific data worries. If you are concerned about what we post about kites being available to the world unencrypted, then maybe you're taking this silly hobby too seriously.
Of course, if you use the same password on this site as on your bank web site or TurboTax or Amazon, then maybe you need to re-think your internet hygiene...