root "su - <user>" got permission denied (Ignored)

Question

Got permission denied when switching user from root to postgres. No issue switching from root to ubuntu.

root@node-01:~# su - postgres
su: Permission denied
(Ignored)
postgres@node-01:~$ logout
root@node-01:~# su - ubuntu
ubuntu@nodeg-01:~$

Here is the /etc/passwd and /etc/group entries:

postgres:x:998:998::/home/postgres:/bin/bash

postgres:x:998:

Answer 1

Problem solved by adding the user to /etc/security/access.conf.

Answer 2

While logged in as the root user type:

"chmod 775 /bin/su"

This changes the permissions for owner and groups to 7 (r,w,x) and the permissions of other users to 5 (r,x) of the 'su' binary file in the /bin directory.