I experience the exact same issue as described here (emqx/emqx#14071) while using this addon. I've tried accessing HA through my own reverse proxy as well as through its native 8123 http interface. Both exhibit the 404 error.
When I go to the web-interface through http:/hostip:18083 it works as expected. Any way this can be rectified? Is this an error on the HA ingress logic (and if so, how/where to raise issue)?