If Nginx is enabled as a reverse proxy service, then HTTPS requests fail to return the full certificate chain.
The SSL certificate checker shows that it is not fully trusted:
It can be checked at http://www.digicert.com/help/.
The case has been submitted to the Plesk Service Team as an internal issue with the ID TP-113865.
The issue has been fixed in Plesk Panel 11.5 MU#14.
NOTE: An SSL certificate should be recreated in the Panel after Micro-Update installation.