I am not an expert in this area, but regardless of whether your custom domain has the security in place, I am certain that as the forum will still be served by Nabble server's then that part will remain insecure and you will get the result you see. See:
http://support.nabble.com/HTTPS-SSL-urgent-td7594627.htmlYou can find further related topics if you search Nabble Support for "https".
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.