Set your system for debug logs (Settings -> Server -> General)
Recreate the problem (play the movie until the error occurs)
Stop playback
Wait 1 minute for server to settle & log everything
Pull logs and attach the entire zip file to the thread (drag it into the compose window). Settings -> Server -> Troubleshooting.
Also, include the Plex XML information for the movie you played. Save the information as a .txt file and attach to the thread (the forum does not allow .xml attachments).