There's definitely a bug in there.
I've assigned a certificate to a domain and two of it's subdomains... Now I want to replace it by antoher certificate, but there's no way to remove it... Always this same message: Unable to remove certificate: one or several certificates are assigned to the IP addresses/domains.
The problem is that to remove a certificate, you need to add a new one, and replace the one you want to remove. But there is no way to UNASSIGN an assigned certificate without replacing it. This is a MISS from parrallels team.
To confirm my theory, I have added my new certificate in plesk with a new name. Then, I have replaced the certificate in my domain and subdomains. After that, I was able to successfully remove the certificate I wanted to delete.
Igor,
Please escalate this to your development team for them to add a "unassign certificate" option for domains and subdomains... This will be most helpful.