HTTPS Error Control Panel on live server

Hi Folks,

I have problems using the control panel on https live server. If i want to, for example, adding a fieldset per cp, this error appears on the browsers dev tool

Mixed Content: The page at 'https://21-raum.de/cp/fieldsets/about' was loaded over HTTPS, but requested an insecure XMLHttpRequest endpoint 'http://21-raum.de/cp/fieldsets/about'. This request has been blocked; the content must be served over HTTPS.

i have already checked the system.yaml. everything seems ok? this is, waht it looks like

license_key: c81c6131-46a4-4ab8-9bd0-017ca8caf9c9
locales:
  en:
    full: en_US
    name: English
    url: 'https://www.21-raum.de/'
timezone: UTC
date_format: 'F jS, Y'
default_extension: md
filesystems:
  content:
    driver: local
    root: site/content
  storage:
    driver: local
    root: site/storage
  users:
    driver: local
    root: site/users
  themes:
    driver: local
    root: site/themes
    url: /site/themes
app_key: n18s0Yo6vvN7kz4udi1LTDL8cFVfI7y5
redactor:
  -
    name: Standard
    settings:
      buttons:
        - formatting
        - bold
        - italic
        - link
        - unorderedlist
        - orderedlist
        - html
  -
    name: Basic
    settings:
      buttons:
        - bold
        - italic
yaml_parser: symfony
send_powered_by_header: true
ensure_unique_ids: true
use_https: true

so what else could id do? Could you please help me?

greetings Patrick