OpenPHT has long since (several years, actually) ceased to be developed further and adapted to the newest Plex features.
It might just be that the latest server has become changed too much for that old client.
Your best bet is to:
- switch the operating system to something like LibreElec (basically stripped down Linux with Kodi as only app)
- install the Kodi Add-On PM4K into Kodi. (now available from the official add-in repository) PM4K / PlexMod for Kodi (18, 19, 20, 21)
