Yesterday, I came to the conclusion all my cron entries were removed (for the second time!) in my /etc/crontab file on my Synology DS213 NAS server.
I noticed the cron entries were removed after a system reboot. I figured out Synology has some problems with cron entries having columns separated with spaces (not tabs). All entries including spaces are removed on system startup.
Updating my crontab by putting tabs (\t) between the columns, fixed the problem.