I can confirm that I had the same issue as @veri745 and their recommended fix above corrected the issue with Plex starting up. This happened right after upgrading Unraid from 6.9.3 to 6.10.2 for me. Thank you @veri745!
Full command for anyone interested
user@host: /mnt/user/appdata/plex# chown -R nobody:root Plex\ Media\ Server/