I’ve installed Plex Media Server on my HA and it’s working fine while i’m connected at home accessing throug this link : http://homeassistant.local:32400/web
nabucasa is a direct link to HA and only HA. Any other port would need it’s own service. If you’re using a plex addon, you should be able to open the web UI through the addon. It won’t be accessible via xxx:32400 externally.