Following the reboot of some cp hosts in eqsin I've noticed that the check Confd vcl based reload that executes via NRPE check_confd_vcl_reload that in turns executes /usr/local/lib/nagios/plugins/check_vcl_reload, returned UNKNOWN with the message NRPE: Unable to read output.
I think that this is due because the script exit without any output:

test -f $STATE_FILE || exit $STATE_OK

If that's the intended behaviour something like this should fix it:

test -f $STATE_FILE || echo "No file to check" && exit $STATE_OK