I have a couple of Samsung tvs that were working totally fine, then out of the blue began having issues where anything attempted to play on plex gave an error of “Unable to connect to the content server” . My Plex Server Security settings is set to “preferred” as you mentioned, and Secure Settings on the Samsung Smart TV App are disabled.
The only way to get them to play anythign and not give the error is to set the Player Playback to “Transcode” instead of “Auto” but that is just so intensive on the server, and those TVs before used to Direct Play pretty much everything!
Not sure what happened from one day to another. Is there anything else I can test or shoudl try?
This is one of them (2012 E-Series with Latest Firmware 2004.0):
This is the other Samsung also with latest firmware: