Ssl – website uses an invalid security certificate (Error code: ssl_error_bad_cert_domain)

You can only have an SSL certificate for a single IP address. You will need to give the subdomain a different IP address and a new SSL certificate tuned to that address.

There is also the option of using multiple domains on your certificate, if your CA supports this. Then you'd just create a certificate for all the domains on that IP and use that.

This might confuse some older clients and be viewed as bad style, though.

You have 2 options.

1. Use another IP as you've mentioned.

2. You will need to manually edit the IIS 7 config from the command line. I've outlined the steps here: link

Doing this will override the default settings of IIS 7 to allow the same cert to be used on different sites with the same IP. It's a common step everyone had to take in the IIS 6-7 days.