Solved 
I overlooked a notification of Google (very small) that a page was trying to load an unsecure page. After allowing to load the page, the webpage of kodi loads.
Did anyone managed to integrate the web-ui of Kodi in a Iframe ? (with https )
What kodi web ui? There are many.
Are you having problems? If so, tell us what they are.
It is possible to open Kodi with a webbrowser : ipaddress:1880
Because I use duckdns I think I need to use a secure url in a Iframe to go to Kodi. Kodi does not support SSL
I want to hide my Kodi-raspberry behind my audio-set and want to choose my playlist in a Iframe
I tried a lot of options: with or without https/ local ip-address / duckdns-url etc.
My home-assistant server is on a Synology Nas in Docker.
When I try something with https I ostly get an error that says : connection refused, when I use http,screen keeps blank
code in lovelace:
- title: kodi2
panel: true
cards:
- type: iframe
url: https://10.0.0.19:1880
aspect_ratio: 75%
username: secret
password: secret