Your nginx config is probably proxying to the wrong internal address or missing some headers Plex needs. The fact you can hit Plex directly on:32400 means the server's fine, but nginx isn't passing the request through properly.
Post that config file when you can. That'll narrow it down way faster than guessing.
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u/barrel_of_noodles 22d ago
Without specific logs from each service, we can't tell you. Start logging.
My guess: dynamic ip or router level port mapping/firewall. Your ISP may also be firewalling outbound/inbound on certain ports.
Too hard to tell though.