Not sure if you are still having issues, but I had very similar issues a few months ago and it was the Amazon firestick. It could not see the server from time to time and I would have to reboot the stick every few days. However plex was running fine on our smart TV, phones, tablets, and WD tv lives boxes. I removed the Firestick, plugged in a chromecast and I haven't had an issue since! if you can..when you have issues, try to connect from another device.