Trying a free SSL cert from Comodo and had some difficulties with an error message:
The CA certificate does not sign the certificate
So downloaded the following CA bundle:
https://support.comodo.com/index.php?_m=downloads&_a=viewdownload&downloaditemid=77
and used that instead of the one supplied in the email and the certificate was accepted.
The install instructions here explain the process for Plesk. If using a single IP address then the certificate will have to be made the default under Server > IP Addresses. So purchasing another IP address will be necessary.
