I guess that the error comes from the change_pass plugin, that needs a poppassd
server running and listening on port 106 by default. This is used to change passwords in /etc/passwd.
The change_passwd plugin on the other hands runs chpasswd to change directly the password in /etc/passwd, so it doesn't need to connect to any other server.
Note that for both plugins you need to install the compatibility plugin also.
I suggest you to try the 2nd plugin that is easier to configure. Just keep in mind, that since you're running RHEL, if SELinux is enabled, it might not let you change passwords using a program ran by the apache user.