HTTPS doesn't work well with mirrored sites.
Why? Because mirroring the site tells our servers "hey, I want the exact same site set up on both of these domains", and that'll make the same SSL certificate be used for both of them. Unless you've got some kind of crazy wildcard or multi-domain certificate, your certificate will only be valid for the main domain, not its mirrors.
If you need multiple SSL sites to run from the same set of files, create a separate fully hosted domain for each one, and set them all to use the same directory. It's kind of a hassle, but it's the only way we've currently got to have them use different certificates… sorry.