When SELinux (the Security-Enhanced Linux feature that provides a mechanism for supporting access control security policies) is enabled or set to permissive mode on the NX Server Manager host machine, accessing  NX Server Manager Web interface may fail with an error:

Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

The Web server error log could report an error similar to:

[error] [client 127.0.0.1] DBI connect('/usr/NX/var/db/manager/nxmanager_conf.db','',...) failed: unable to open database file(1) at dbdimp.c line 92 at nxmanager.pl line 1432

This is due to SELinux blocking NX Server Manager. In order to allow NX Server Manager working properly, it is necessary to configure SELinux to allow its operations. Some instructions related to NX Server, which could be extended to NX Server Manager too, are available here:

https://www.nomachine.com/AR05D00389

Another possible solution is to allow NX Server Manager to run without SELinux protection. To do this, run the following command from an xterm or similar as privileged user:

# chcon -t httpd_unconfined_script_exec_t /usr/NX/bin/nxmanager

Excerpt from:  http://fedoraproject.org/wiki/SELinux/apache

httpd_unconfined_script_exec_t
- Set cgi scripts with httpd_unconfined_script_exec_t to allow
them to run without any SELinux protection. This should only be
used for a very complex httpd scripts, after exhausting all
other options. It is better to use this script rather than
turning off SELinux protection for httpd.