A new task started in Plesk hangs (e.g. subscription creation, scan in WordPress Toolkit, update of extensions, etc.).
The following error message appears in the Plesk Task Manager log
ERROR task=324 process=324 trace=7247:60da61c35a000 Start failed: Failed to start the process: exit status 1 (/usr/bin/systemd-run: unrecognized option '--quiet')
The server is running Debian OS which has been recently upgraded from Debian 8.
systemd package has not been upgraded during the dist-upgrade from Debian 8. The installed package version is for Debian 8:
# apt-cache policy systemd | grep Installed
Connect to the Plesk server via SSH.
# apt-get install systemd -y
Remove the information about hung tasks from the Plesk database:
# plesk db "DELETE longtasks, longtaskparams FROM longtasks LEFT JOIN longtaskparams ON longtasks.id = longtaskparams.task_id WHERE longtasks.status = 'not_started';"