There are many errors that can appear when Plex Media Server attempts to transcode a media file. Most transcoding errors that are displayed don't provide much information as to the cause of the error, so it is up to the server owner to try and resolve the issue.
An example of one such Plex playback error message is:
Conversion failed. The transcoder exited due to an error.
If you get this message when streaming a movie that is being transcoded, then there are a few things that you can try to resolve the issue.
Update to the latest version of Plex
The first thing to try is to ensure your server is running the latest version of Plex Media Server. Most releases of Plex tend to provide fixes to problems that users are having, so updating to the latest version could solve the issue.
While you are updating Plex, you should also check for updates to your operating system, just to be safe.
Check the location and permissions of the transcoder temp directory
Verify that the location of the temporary transcoder directory is in a directory that the user running the Plex server has write access. Plex writes out blocks of the media file as it transcodes so it can stream the file to the client machine, and these blocks are written to files in a temporary directory.
If Plex can't write to the directory, then this could cause this error. Ensure the directory is accessible and writeable by the user running Plex.
Many times transcoder errors can be the result of trying to burn subtitles in the movies. Try disabling subtitles to see if the error disappears.
If this resolves the issue, check for missing or bad subtitle files that are associated to the movie, or simply replace the subtitle files with new, good versions and try enabling subtitles again.
Check if volume is mounted with nonexec
The drive that contains the Plex transcoder directory must be mounted with the "exec" option. If you have installed Plex on a drive mounted with "nonexec", this means that the Plex transoders will not execute, which will result in the transcoder exiting error.
To resolve this issue, you will need to go into /etc/fstab and manually remove the "noexec" parameter, or change it to "exec" to get the Plex to transcode again.
If the above three solutions didn't fix the codec issue, then the next step would be to try asking for solutions on the Plex forums.