Hi tiaar,
to check the system related cronjobs on your system and how they are actually defined for the system user "root" for example, please use the command:
crontab -l -u root
To make it easier for Plesk users to handle cronjobs, the above link from
IgorG shows the Plesk utility "Scheduled Tasks" and you may always check the logs:
Ubuntu/Debian:
grep CRON /var/log/syslog
tail -f /var/log/syslog | grep CRON <= command to monitor the current syslog, use "CTRL + C" to quit.
CentOS/RHEL:
Logfiles : /var/log/messages // /var/log/cron
For investigations, what might be the cause of your issue, please try to include errors from log - files and describe the exact way, how you tried to set up the cronjob.
Another reason, why cronjobs may not work as expected, is the usage with SELinux on your system, being poorly or inadequately configured.