I finally understood that I have to use the “NGINX Home Assistant SSL proxy” Add-on and why it is so. 
The reason was to be able to use the Android Companion app (which doesn’t work from LAN with DuckDNS + LE)
Now I’m facing another problem: I can only access HA from LAN with http - so I guess NGINX is doing what it’s supposed to.
But, when trying to connect from a foreign network I get following errors:
And after pressing “RETRY”:
Oops.
The site at https://xxx.duckdns.org/lovelace has experienced a network protocol violation that cannot be repaired.
EDIT: everything was working as expected before installing the NGINX Add-on
