Plesk upgrade fails: Failed to list unit files: Connection timed out

Symptoms

Plesk upgrade fails:

PLESK_ERROR: Trying to start service mariadb…
WARNING! Some problems are found during attempt to start service mariadb – control script doesn’t exist or isn’t executable(see log file: /var/log/plesk/install/plesk_17.0.17_installation.log)
Starting sw-engine (via systemctl): Failed to start sw-engine.service: Connection timed out [FAILED]
Trying to start service drwebd…
Failed to list unit files: Connection timed out

Cause

Services fail to start because of high CPU usage by systemd.

Resolution

1. Connect to the server via SSH.

2. Create a directory:

   # mkdir /root/example

3. Move session files from /run/systemd/system:

   # mv /run/systemd/system/session-*.scope* /root/example/

4. Restart services from the error message:

   # service sw-engine restart
   # service drwebd restart

5. Restart the upgrade.