Cron recently started to ignore any changes to
/etc/crontab. Cron's mail works for the older entries. Restarting cron with
sudo systemctl restart cron has no effect. I did not install new packages since it last worked properly. I do not want to restart my server to find out if that helps; that would just be a workaround anyway...
I use Debian 9.6 and Cron 3.0pl1-128+deb9u1.