I just installed rasplex on a raspberry pi 3, it had me enter a pin that I got from plex.tv/pin or something similar and I can see it as a device when I go to the server (also running on a pi 3) directly in a web browser on a PC on the network. I did that, went to the IP x.x.x.x/32400 and logged out with the intention of logging back in thinking that may help the rasplex see the server.
Now the LOCAL server doesen't see itself. I am logged into x.x.x.x:32400/web and its trying to find a plex media server. How does that make sense? It is the plex media server.
From searching it seems the best advice is to log out of the server itself but thats what I did.