Script in "cron.d" not starting: Authentication failure

[^rooker]

[PROBLEM]
I've added a crontab-style config file to /etc/cron.d, but it won't get executed.
The only traces I find in the logs are:

/var/syslog:

Jun 24 15:50:01 hostname /usr/sbin/cron[8337]: (*system*sensor_graphs) RELOAD (/etc/cron.d/sensor_graphs)
Jun 24 15:50:01 hostname CRON[10650]: Authentication failure

The only indication that the Authentication failure belongs to my script "sensor_graphs" is after I modify something, because then cron writes the "RELOAD" comment to the logs.

[SOLUTION]
Inside the a crontab file in "/etc/cron.d", I've specified a non-priviledged user. Its password was disabled (like all other system users, too), but it was missing a corresponding line in /etc/shadow.
I've added that line - and voila: Now it works!