Please see the following thread:
https://www.nomachine.com/forums/topic/authentication-failed-with-error-6
If that doesn’t help, please send the output of the following to issues [at] nomachine [dot] com:
1) /etc/pam.d/nx, /etc/pam.d/su, /etc/pam.d/system-auth, /etc/pam.d/sshd, /etc/pam.d/password-auth files
2) output of the command (after reproducing the issue):
sudo grep -r nxexec /var/log/*
Thank you.