All users can not login on AIX machine
Wrong configuration of the etc/security/user file. In the default stanza there was the entry registry = compat
Edit the /etc/security/user file and remove the entry registry = compat
on AIX, there is SYSTEM, and registry.
SYSTEM controls authentication, registry controls identity.
default stanza tells all users without a specific entry how they should work.
Setting registry = compat in that stanza tells ALL users their information comes from the local BUILTIN module.
As apposed to determine it based on what SYSTEM auth'ed them.