This usually is a filesystem or file corruption issue....A few things to check:
The filesystem(s) that hold /etc, /var and /tmp have disk space available
The /etc/passwd, /etc/group, /etc/security/user, /etc/security/login, /etc/security/passwd and /usr/lib/securty/methods.cfg have correct permissions
On later AIX try running "userck" to check the validity of the above files
If you have an earlier version of AIX, check for blank lines in /etc/passwd and /etc/group, and look for double blank lines or corruption in /etc/security/users, /etc/security/login and /etc/security/passwd
Is your system configured to authenticate against an external security database, for eg NIS or LDAP ?