Keep running into this problem. Getting cron failures in /var/cron/log:
someuser 4151 c Mon Feb 29 12:35:00 1993
! bad user (someuser) Mon Feb 29 12:35:00 1993
Inevitably, the user has locked themselves (or a malicious someone) out of the server.
Check /etc/shadow for *LK* on the user.
passwd -u someuser to unlock.