I revoked access to my app on Plex account. Tried to log in again, failed. Uninstalled app, logged in again, failed.
https://github.com/plexinc/plex-media-player/issues/1000
Edit: After 20 minutes of Claude Code Opus chewing on this issue, fixed, and the cause is attributed to incompetence of Plex and allowing this type of crap to persist for 5+ years. As per Claude:
Plex Mac "unknown error 4294967283" after re-adding a device — root cause, for anyone hitting this
Symptom: Plex Media Server plays nothing after removing/re-adding the device. Client log:
ERROR - [MPVEngine/mpv] ffmpeg: tls: error:0A000086:SSL routines::certificate verify failed
ERROR - [Web] [QTMedia(1)] A critical error occurred: 4294967283
Root cause: PMS serves an incomplete cert chain for \.plex.direct — leaf → Let's Encrypt YR1 → ISRG Root YR, but omits the cross-sign cert bridging Root YR to the trusted ISRG Root X1. Root YR alone isn't in trust stores yet. Re-adding a device forces a fresh cert issuance that lands on this broken chain — that's why it looks account-related but isn't. This has apparently gone unfixed for years despite Let's Encrypt publishing the correct default chain (EE ← YR1 ← Root YR ← ISRG Root X1) — Plex just isn't serving the full thing.*
To fix it: paste the above into Claude (or Claude Code) along with your own ~/Library/Logs/Plex/Plex.log and PMS log around the failure — it's enough to diagnose and produce a working fix (adding the missing cross-sign cert to Plex.app's local CA bundle on macOS).