Can´t get SSL / nginx working together with local access

Installation
1

I know the are already many many threads about this topic. But I read a lot of them and still can´t get my HA working as expected.
My Setup is pretty simple: HA OS is running on a odroid-n2 behind a Fritz!Box.
Portforwarding in the router is set up for port 443.

I´ve installed DuckDNS with this config:

domains:
  - xxxxxx.duckdns.org
token: xxxxxxxx
aliases: []
lets_encrypt:
  accept_terms: true
  algo: secp384r1
  certfile: fullchain.pem
  keyfile: privkey.pem
seconds: 300

Nginx is also set up:

domain: xxxxxxxxxx.duckdns.org
hsts: max-age=31536000; includeSubDomains
certfile: fullchain.pem
keyfile: privkey.pem
cloudflare: false
customize:
  active: false
  default: nginx_proxy_default*.conf
  servers: nginx_proxy/*.conf

Now to the issue I have: I didn´t find a config.yaml that allows me to access HA from the local network and from outside via SSL. I only get one of the two working at the same time.

With this config.yaml acess only SSL access via https://xxxxxxxx.duckdns.org works:

http:
  use_x_forwarded_for: true
  trusted_proxies:
    - 127.0.0.1
    - ::1
    - 192.168.0.0/24
    - 2001:DB8:ABCD::/48
  # Paths to your Cert Files
  ssl_certificate: /ssl/fullchain.pem
  ssl_key: /ssl/privkey.pem
  # IP Ban
  ip_ban_enabled: true
  login_attempts_threshold: 5 # the number of failed logins before banning an ip address

with the SSL part commented out only local access via http://homeassistant.local:8123/ works:

http:
  use_x_forwarded_for: true
  trusted_proxies:
    - 127.0.0.1
    - ::1
    - 192.168.0.0/24
    - 2001:DB8:ABCD::/48
  # Paths to your Cert Files
  #ssl_certificate: /ssl/fullchain.pem
  #ssl_key: /ssl/privkey.pem
  # IP Ban
  ip_ban_enabled: true
  login_attempts_threshold: 5 # the number of failed logins before banning an ip address

Anybody got an idea what I am doing wrong?