When I get a connection refused error its normally caused by another program pinching port 1248 that nsclient uses, exchange does this alot, sql might also be similar.
If the config worked then all of sudden you got connection refused this is normally the cause.
- reboot server (not always an option)
- netstat -a (or whatever allows you to see the PID) match the PID listening on port tcp 1248 with the PID of nsclient if they match it could be the nsclient has crashed, if they dont it means another program has pinched the port.
- stop the nslcinet in services.msc it will probably fail. Kill the psnclient.exe process wait about 5 minutes and start it again, this should work. If not stop the other service/program using the 1248 port and start nslclient and then start the other service/program.
- You could also try uninstalling and re-installing the nsclient to. (stop it first though!)
I'm monitoring about 85 hosts on one of our nagios box and this happens to a host at least once a week.
hope thats of some help