Adguard home - login via https proxy manager (nginx)

So, I have adguard home running on a separate ubuntu server.

this works:
http://localip
internal dns like http://aname.mydomain.com
internal dns via nginx reverse proxy manager https://aname.mydomain.com

all prompts me for the username password and logs me in.

When I add this to my homeassistant panel_iframe.yaml:

  adguard:
    title: AdGuard Home
    icon: mdi:shield-check
    url: https://aname.mydomain.com

I shows me the same login page but the credentials are not accepted, I cannot login that way.

What am I missing there?

UPDATE: it does work on my mobile companion app…

Oh no… am I hitting a bug here?