In case the certificate is self-signed, or the certificate is issued by an Enterprise CA, Parallels Clients should be configured as follows:
Export the certificate in Base-64 encoded X.509 (.CER) format.
Open the exported certificate with a text editor and copy the contents to the clipboard.
To add the certificate with the list of trusted authorities on the client side and enable Parallels Client to connect over SSL with a certificate issued from an organization’s Certificate Authority.
On the client side in the directory "C:\Program Files\Parallels\Remote Application Server Client\" there should be a file called trusted.pem. This file contains certificates of common trusted authorities.
Paste the content of the exported certificate (attached to the list of the other certificates).