bas luck for you: this error is a generic error message... it doesn't mean anything (so stop searching in google for this error code :)).
The thing it may mean, is that you have a problem with your plugin => nagios isn't able to execute it properly.
So, check the rights on the plugin, check that all your arguments are passed, check that you have no "relative paths" in your plugin (ie: ".../my_path" instead of "/usr/my_path" for exemple).
Don't hesitate to add "log points" in your plugin (if it's a shell script, try some "echo $VAR >> /tmp/my_log_file.log" at several points in your script, so you'll know what is really going on, from the nagios execution view).
Sorry I can't be more specific, but this error message is quite un-specific.
ps: don't hesitate to post your plugin/script if you really can't find the issue