I've had this kind of errors for 2 entirely different reasons:
- on a server, we were reaching the limit of the maximum number of processes launcheable by one user (nagios). ie: this limit was fixed a 120 processes, and we were above this limit, thus this error. We had to increase the limit on this server (don't know how to do that though, so we had to ask this limit to be increased :))
- the check_ping plugin was "not well" compiled for this server; in fact, we imported the check_ping from another server which should have been exactly the same ... in the end, this plugin was not working correctly, and we had this kind of errors around 0.3% of the time. As we couldn't compile on this server, I had to write a small .pl script that would call /usr/bin/ping and parse the result (which is what the check_ping plugin does, anyway).
I hope this will help you;
if you're not in one of theses cases, I can't help you further :/