It has been verified that when nscd  (Name Server Cache Daemon) is running on the host machine, some delay in updating the cache can occur. 

In this case, even if NX server has been previously correctly uninstalled, and the user nx removed from the system, the 'id nx' can still be present in the cache and your next attempt to install NX server can fail with error:

NX> 704 ERROR: Cannot add user: nx. User: nx already exists.
NX> 704 ERROR: Please try to fix the problem by reinstalling the server.

In a similar way, it could also happen that the nx user has been created on the system during the NX Server installation, but the cache is still not updated. In this
case, the 'chown' command issued during the installation procedure could fail, for example with error:

NX> 700 ERROR: Output: chown: unknown user id nx.
NX> 700 ERROR: Cannot set ownership attributes for
'/usr/NX/etc/passwords.db.lock' to 'nx:root'

In order to solve both these issues, you may try to stop and restart the NSCD process:

# /etc/init.d/nscd stop
# /etc/init.d/nscd start