Some media codec looks like have been retired… there are many posts about similar issues. For example older AVIs or media encode with older DIVX codecs don’t play… and the AV1 container has never been supported in Plex… yet.
https://forums.plex.tv/t/plex-ms-avi-problems/628030
Other issues… your server maybe trying to encode something that it needs to transcode because of the audio. There is a trick using RAM disk to help eek out a little more performance that may aid.
[Guide] Plex Transcoding and RAM Disks - Software - serverbuilds.net Forums.
I’ve requested the error message be more informative to help aid with diagnosing the issues when it comes up.
good luck.
