Hi,
on my Debian Buster the crontab line from the admin manual does not work at all. The new cron.php tries to execute ./occ, but the pwd of the cron job ist not the owncloud directory. My solution till the next release is the following crontab line:
*/15 * * * * cd /var/www/owncloud && /usr/bin/php -f /var/www/owncloud/cron.php >/dev/null 2>&1
The redirection at the end sends all messages to /dev/null and thus no spam is produced.
Regards Jochen