SSL is already being used by www.domain.com which shares the same IP address, xx.xx.xxx.xx, as this site. SSL can only be enabled for one site using a shared IP address. Change the IP address of this site or disable SSL for www.domain.com if you want to enable SSL for this site. What does this mean exactly?
Just what it says - the virtual hosting only allows for one site per IP to have SSL enabled, it is VERY limiting in that regard. If you only have 16 IPs, you get 16 sites with SSL.
At least on our RAQ3iJale