which update? web ? server ? version? previous version?
see note below regarding diagnostics
would need to have plex web debug logging enabled (beforehand) and also the Plex Media Server debug logging and capture the diagnostics when the error occurs with screenshot (full - not cropped) and noting time of the error and then collecting the Plex Web log and the server logs zip
see
https://support.plex.tv/articles/201611836-plex-web-app-logs
https://support.plex.tv/articles/201643703-reporting-issues-with-plex-media-server/
https://support.plex.tv/articles/200250417-plex-media-server-log-files/
