PMS version?
1.41.7.9823 & 1.41.8.x should transcode TrueHD to EAC3 (as should 1.41.6 & earlier).
1.41.7 introduced a bug transcoding to Dolby formats that caused playback to fail.
This affected the Plex LG & Roku clients, as Plex Media Server normally transcodes TrueHD to EAC3 for them (other clients may also have been affected).
The problem was fixed in 1.41.7.9823: Plex Media Server - #677 by chris_decker08
You must also disable Force Direct Play if enabled in the Plex client app.
