Finally (the second time I tried), the article https://support.plesk.com/hc/en-us/articles/360012999480-Email-notifications-are-received-daily-from-a-Plesk-server-etc-cron-daily-imunify360-firewall-exited-with-return-code-127 helped me out, with the following command:
on Debian/Ubuntu-based...
Hello Igor, thank you. It helped me to remove the existing cronjobs from Imunify, and the error messages about those cronjobs by e-mail since I deleted Imunify from Plesk.
The only message I keep receiving is:
/usr/bin/flock -n /var/run/shrinker.lock /opt/alt/python38/bin/python3...