it could be a timeout of the request to load the dashboard content.
A database optimization has been seen to resolve this for some users - see this support page
https://support.plex.tv/hc/en-us/articles/201420066-How-can-I-make-On-Deck-appear-faster-
If this does not resolve the problem, then would need plex web app log and also Plex media server.log
See
https://support.plex.tv/hc/en-us/articles/201643703-Reporting-issues-with-Plex-Media-Server
https://support.plex.tv/hc/en-us/articles/200250417-Plex-Media-Server-Log-Files
https://support.plex.tv/hc/en-us/articles/201611836-Plex-Web-App-Logs
