To start with would need some server logs. After the initial investigation, it may become necessary to ask for a short clip of the media file that also produces the error
Ensure you are running on the latest release
See https://plex.tv/downloads
Ensure debug logging is enabled
see https://support.plex.tv/articles/201643703-reporting-issues-with-plex-media-server/
Restart the server to get fresh log files created
Reproduce the problem
Note down the time of error and what was being streamed
Download the logs
See https://support.plex.tv/articles/200250417-plex-media-server-log-files/
Attach your logs zip file to a post here with info on when the error arose and what plex client app and version was being used
