I’m not familiar enough with cron (!) but after several tests, it seems
that it doesn’t work correctly.
In fact, I don’t know why I saw some entries working, but the backup I
want to do every Friday never works!
no logs in /var/spool/cron nor in /usr/spool/cron.

my crontab is:
0 0 * * 5 /root/admin/

I have a cron.allow with only root declared.