The issue is likely due to you listening on multiple interfaces on your FreeNAS box. I had the same Plex server not found problem on FreeNAS 11.1 in a jail, and resolved it by adding my internal network and netmask to the /usr/local/plexdata-plexpass/Plex Media Server/Preferences.xml file, like this. It’s XML formatted data, so just stick it before the last /> in the last line. Restart plex, and boom, server not found is fixed.
allowedNetworks="192.168.0.0/255.255.254.0"
