If I understand your error description correctly, then the red URL is probably confusing that you are using SSL and have an invalid or self-sign certificate.
However, this should not affect the plugin installation. You can try to download the plugin from Marketplace (from this link) and plugin install manually (Confluence administration -> Manage apps -> Upload app).
Pavel
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.