It could be the issue reported by other users which has been tracked down to a bug in code that handles loading of iTunes media / library into Plex.
See Media Server is unreachable - #17 by sa2000
If you do not includes iTunes media in Plex please provide logs
See
https://support.plex.tv/articles/201643703-reporting-issues-with-plex-media-server/
https://support.plex.tv/articles/200250417-plex-media-server-log-files/
