to stop and start “crontab” in the following (non-standard-Linux) way (from forum.synology.com):
/usr/syno/etc.defaults/rc.d/S04crond.sh stop
/usr/syno/etc.defaults/rc.d/S04crond.sh start
or Reboot the NAS: Remember Rebooting the NAS will take 5~6 Min..