If the logs of your Apache Web server report an error similar to:

...Connection to NXManager DB failed: unable to open database file...

it is likely the files needed by the NX Server Manager to handle the statistics have wrong ownership and/or permissions. In this case, you should update them according to the following instructions:

# chown -R user:group /usr/NX/share/htdocs/
# chmod -R ug+rx /usr/NX/share/htdocs/

# chown -R user:group /usr/NX/var/db/nxmanager
# chmod -R 0760 /usr/NX/var/db/nxmanager/

# chown -R nx:root  /usr/NX/var/db/nxstat/
# chmod -R 0770 /usr/NX/var/db/nxstat/

where user and group should be replaced with the user and group qualifying your Apache user for example:

# chown -R www:www  /usr/NX/var/db/nxmanager