The gross incompatibility between new (2.9+) nsca clients and old (2.7 and earlier) nsca servers described in #670373 means that nsca on the monitoring server has to be upgraded before nsca-client on any machine that is configured to submit results. I have just prepared and uploaded a backport which should make the situation easier for admins who don't want their monitoring system to be the first to be upgraded to wheezy. Cheers, -Hilko