It could be a problem with permissions. Can you send us the output of the following command?
ls -laR /etc/NX
You can fix permissions of files in the directory /etc/NX/server/localhost by executing:
sudo chmod 644 /etc/NX/server/localhost/*
If this doesn’t work, then send us what’s in /usr/NX/var. You can send it to issues[at]nomachine.com.