If you are using custom domain to your subdomain with CNAME, you will need a separate certificate for that custom domain.
HTTPS protocol is valid for all *.helpjuice.com subdomains, and when you're using CNAME your domain HTTPS is no longer valid for *.custom_domain.com. HTTPS still exists on your KB login page and it exists when you don't use CNAME, but when you do use CNAME the HTTPS isn't shown.
To upload your custom certificate for your custom domain, follow the steps in this article.
The less secure option is disabling Force SSL feature in Settings page (image below) and data encryption won't be forced.