While setting up several new servers and installing NSCLIENT, I ran into the following error message:

could not fetch information from server

The most logical first step is to re-verify the Nagios server config file.  Check to make sure DNS resolution is correct.  Second, take a look at the NSC.log on the client system.  In my case, I saw:

2009-03-30 10:52:23: error:.\NSClientListener.cpp:307: Unauthorized access from: 172.20.16.182

Well, that could definitely be a problem.  My fault this time was in editing the NSC.ini after installation.  The allowed_hosts line of:

allowed_hosts=172.20.16/23

needed to be like:

allowed_hosts=172.20.16.0/23