As described here https://github.com/naemon/naemon/issues/70 this seems to be a nrdp issue. Setting the `early_timeout` to `0` solves the issue for Naemon: https://github.com/NagiosEnterprises/nrdp/blob/6ee9a750b21659cd1cff38eecb5ddb4097156108/server/plugins/nagioscorepassivecheck/nagioscorepassivecheck.inc.php#L222