I'm sharing this as it caught me out for a few days and the solution wasn't obvious.
I run Plex to serve media around the home to my TV and few
LAN and Wi-Fi clients. The server runs on a Mac Mini and I keep up with updates.
The client running on the server was fine but suddently LAN and Wi-Fi clients could
no longer connect, even when manually specifying the server IP. Server logs didn't
show any errors but it did look like the clients were no longer managing to connect
or query. It seemed that going to
http://server-ip-address:32400 should at least
show something in a browser or the logs.
I wasn't using myPlex and it didn't appear as if they were forcing it in a recent release. I was eventually tipped off by this comment in a server release changelog:-
(Also, we've updated the Plex for OS X download to fix the code signing issue that was causing the violent disagreement between it and the OS X firewall. Sorry about that!)
I deleted the entries for 'Plex DLNA Server' and 'Plex Media Server.app' from System Preferences -> Security & Privacy -> Firewall -> Firewall Options, restarted the media server and clicked 'allow incoming connections' again for each. As if by magic all my clients could connect again!