I am currently facing an issue with Plex on Xbox one, it is detecting the external servers.
But it indicates that the server on the same network is offline. The Plex overview page displays the server as online. There is also no issue with applications on other devices (Android etc..)
Are you sure you have the right version? Anything lower than 0.9.10.3 (not just any 0.9.10) won't work. I had that issue as well and reinstalled to resolve it.
Same problem here. I'm using correct version of server and XB One can see my server. But the Plex App on XBox says server is offline, even though it's not.
Curious, is your X Box connected through some sort of range extender? I'm thinking some kind of network issue and I have a fairly complicated network.