I'm having some trouble today with deluge, the daemon won't start, not sure why. I'm using a RPi4 in headless with an external HDD. Everything else works fine.
Launch deluge-gtk, go to Edit -> Preferences -> Interface and disable classic mode, then restart deluge-gtk. Add the headless server to the connection manager, ...
Had you ever restarted your NAS since first enabling the WebUI? Its possible the config change didnt save, and so Deluge is not running the WebUI daemon.