There is a problem with DVR Settings on the new DVR Setup ux for DVRs with old lineup that no longer exists (eg TiVo old lineup from 2020)
This is with the Plex Web team at the moment to resolve
There is a workaround posted in the forum here but it involves editing the database
Workaround: There was an unexpected error when going to settings page - Live TV & DVR - Plex Forum
