There was an unexpected error loading the dashboard -

General Discussions
4
  1. activate debug logging (not ‘verbose’!)
  2. quit Plex Server
  3. wait 1 minute
  4. start Plex Server
  5. wait 5 minutes
  6. fetch log files and attach them here

Or inspect them yourself. Take a look at the Plex Media Server.log file and seek for messages about database corrupt or malformed.
If you find these, you may have to repair your database.
https://support.plex.tv/hc/en-us/articles/201100678-Repair-a-Corrupt-Database