I checked the error “nxexecPAMCheckCredentials: Authentication failed with error 28.” on the nxerror.log which I think is the hint for the problem (at least mine). The /var/log/messages shows:
Jan 9 08:49:25 server nxexec: PAM unable to dlopen(/lib/security/pam_unix2.so)
Jan 9 08:49:25 server nxexec: PAM [error: /lib/security/pam_unix2.so: undefined symbol: pam_syslog]
Jan 9 08:49:25 server nxexec: PAM adding faulty module: /lib/security/pam_unix2.so
NX added a PAM configuration in /etc/pam.d/nxsshd which look fine for me. From what I understood about PAM it should be able to use local /etc/passwd,/etc/shadow, NIS and LDAP.