could not open session with "su - user"
As I found out why Linux Redhat 7.1,7.2 and 7.3 servers sometimes display the message "could not open session" when you enter "su - user", I think it should help you.
To debug this, you have to modify one file, named "su" in /etc/pam.d
You should replace, in the lines begined by "auth", "account","password" and "session", the word "required" with the word "optional".
It's the only way to rid off this issue.
Please be careful. Security holes may occur.
PS : please, forgive me for my bad english.