I ran into an error with Transcoding on my PMS running on a Raspberry Pi4.
It runs the latest raspberryPi OS and the latest PMS version.
The HDD was mounted via the /etc/fstab file. the exec Option was set, but transcoding Error remained.
I solved the Problem with a hint i found after hours of searching in this Post: Plex Transcoder Not Working after Application Support folder move - Raspberry Pi
The Exec-Option needs to be the the last in the list. After i made this tiny change i was able to convert and transcode.
UUID=<UID of HDD> /mount/point <filesystem e.g. ext4> defaults,auto,users,rw,nofail,exec 0 0
I hope this post will help someone in the Future