An attempt to open Parallels Installer from Panel at Tools & Settings > Updates and Upgrades or Home > Add or Remove Components fails with the following error:
Command: ai.exe Arguments: Array (  => --service-mode=components  => --enable-xml-output  => --ssl-cert  => C:\Program Files (x86)\Parallels\Plesk\admin\conf\httpsd.pem  => --branch  => release,stable )
Details: file does not exist or is not executable
Another symptom is that the server has not been updated for some time (and there are no notifications about new updates being available).
The problem is caused by a missing Parallels Installer binary (ai.exe).
Download the Parallels Installer binary from our website and put it into the
%plesk_bin% directory (e.g., C:\Program Files (x86)\Parallels\Plesk\admin\bin).
Then run Plesk Reconfigurator and fix the permissions on Plesk files (Plesk Reconfigurator > Repair Plesk Installation > Plesk File Security).