The server is not powerful enough to convert video?

Server Version#: qnap 4.12.3
Player Version#: plex for samsung 4.19.2

I have been using plex servers on my qnap hs 453dx 16gb ram for almost a year
and the Samsung TV UE49NU8000, for a few days I can no longer see the SD files (error The server is not powerful enough to convert video) I have problems with some FHD while the 4K work perfectly. The cpu never goes above 20% obviously it’s a codec problem … I installed the latest version of the server for qnap. I reinstalled the player on the tv without success, what else can I do?

4k FHD sd