Just to compare: I have the ISP’s modem and an Asus router connected to one of the modem’s LAN ports with PPoE (Asus router uses authentication). Do you by any chance have a similar configuration?
Same here. This message has showed up a few times over the last couple of days. Each time I would see it (under Settings-> Server), I would reboot my PC, check again, and it would show that everything is OK.
But just now, when seeing the error message again, I tested by using my 4g connection on my phone, and was able to play something just fine.
I resolved the above by manually doing port forward on my Asus router: I have set manually the port in the Plex server (port 25000) and then did the port forward to the Shield’s IP address /port 32400. Be careful so as to set the Shield’s IP with mac address reservation in the router.