Nagios Support Forum

Did as you suggested and got nothing in the log.

There isn't any NATing going on inside our network.

Weird situation. Especially when it works from the Test XI and not the Prod XI. For Memory and Disk but CPU works on both!!

[nagios@TESTXI ~]$ /usr/local/nagios/libexec/check_nt -H SERVERIP -s "" -p 12489 -v USEDDISKSPACE -l C -w 90 -c 95
C:\ - total: 60.00 Gb - used: 27.44 Gb (46%) - free 32.56 Gb (54%) | 'C:\ Used Space'=27.44Gb;54.00;57.00;0.00;60.00
[nagios@TESTXI ~]$ /usr/local/nagios/libexec/check_nt -H SERVERIP -s "" -p 12489 -v MEMUSE -w 90 -c 95
Memory usage: total:7009.87 MB - used: 1244.81 MB (18%) - free: 5765.06 MB (82%) | 'Memory usage'=1244.81MB;6308.88;6659.38;0.00;7009.87
[nagios@TESTXI ~]$ /usr/local/nagios/libexec/check_nt -H SERVERIP -s "" -p 12489 -v CPULOAD -l 5,90,95
CPU Load 0% (5 min average) | '5 min avg Load'=0%;90;95;0;100

[nagios@PRODXI ~]$ /usr/local/nagios/libexec/check_nt -H SERVERIP -s "" -p 12489 -v USEDDISKSPACE -l C -w 90 -c 95
Free disk space : Invalid drive
[nagios@PRODXI ~]$ /usr/local/nagios/libexec/check_nt -H SERVERIP -s "" -p 12489 -v MEMUSE -w 90 -c 95
could not fetch information from server
[nagios@PRODXI ~]$ /usr/local/nagios/libexec/check_nt -H SERVERIP -s "" -p 12489 -v CPULOAD -l 5,90,95
CPU Load 0% (5 min average) | '5 min avg Load'=0%;90;95;0;100

With the plugin failing on the Prod server and working on the Test server seems to me to be something is filtering it.
You said earlier that there was a security server that was routing the data through it. Make sure that device was not added back.

Lets try to run the nmap command again from the Prod server. Run the following like below and post the full output. The IP address is the Windows system. Next, if the check_nt plugin it not working, let's try the check_nrpe plugin.
Run this command and post the output. The IP address is the Windows system. It should print the NSClient++ version. If it fails, run this nmap command and post the output to see if the port is open.