Symptoms

Plesk shows 502 Bad Gateway error and service 'sw-engine' is marked as stopped.

Cause

This may occur while upgrade attempt was unsuccessful and 'sw-engine' service was not started after it.

Resolution

Make sure that sw-engine is running:

    # service sw-engine start
    Starting sw-engine-fpm:                                    [  OK  ]

Or start psa service:

    # service psa start