Cannot access API from external URL in android mobile app

I have troubles accessing immich API from the android mobile app.
Here is the relevant entry in my Caddyfile:
immich.domain.com {
    tls internal {
        on_demand
    }
    reverse_proxy immich_proxy:8080
}

Here is the relevant .env line
IMMICH_API_URL_EXTERNAL=https://immich.domain.com

immich.domain.com/api/server-info/ping
and
my.docker.host.ip:2283/api/server-info/ping
both gladly 😄 respond
{"res":"pong"}

What am I missing?
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