Please could you provide logs of the problem
Ensure debug logging is enabled
See https://support.plex.tv/articles/201643703-reporting-issues-with-plex-media-server/
Restart the server and when you get the problem please note down the time, take a screenshot and download the logs and attach the zip and screenshot in a post here
For logs See https://support.plex.tv/articles/200250417-plex-media-server-log-files/
And please attach the screenshots here in your forum post and do not put it on a web site that insists on installing loads of third party cookies to whoever wants to view the screenshot
