I found this for you, too. It comes WITHOUT WARRANTY, try at your own risk, please. It is purely an operating system task and has nothing to do with Plesk.
The issue can exists due to a high load on boot. It is also possible that during operations the inter process communication gets stuck (for unknown reason). Check /var/log/messages and /var/log/secure for entries that contain "pam_systemd(crond:session)". If there are also entries like "Failed to activate service", try this:
Reload daemons
# systemctl daemon-reload
Then check health status of inter-process communication daemon
# systemctl status dbus.service
# systemctl status polkit.service
If the dbus service is not running, then you need to reboot your system. Dbus cannot be restarted as a service because it is tied to cgroups which are only created at boot. Verify that dbus is running:
# systemctl status dbus.service
If polkit service is not running, start polkit service
# systemctl start polkit.service
If login service is not running, start login service
# systemctl restart systemd-logind
(but probably that is running, else you would not be logged on, right?)
After these steps your system should operate normally again.