Suggest getting server logs with debug logging
See
https://support.plex.tv/articles/201643703-reporting-issues-with-plex-media-server/
With debug logging enabled, restart the server to get fresh logs created
And on next occurrence of the problem, note down the time and get the logs - either manually by copying logs out and zipping or restart the server and use the web interface to download the logs zip
See
https://support.plex.tv/articles/200250417-plex-media-server-log-files/
