hchan, it looks that authentication stage simply hangs and it is killed after a 20 seconds timeout. Do you have SELinux or similar protection running on your server? You can try to temporary set SELinux in permissive mode using the command:
# setenforce 0
Geekazoid, we are investigationg your issue to find why the load of PAM module fails. Could you check the PAM configuration of command “su” (look at files /etc/pam.d/su or /etc/security/pam.d/su) and if the “su” command works as expected?