Drop this, it does not prevent installing akonadi-backend-mysql.

roles/lmn_kde/tasks/main.yml

@@ -1,13 +1,4 @@
 ---
-- name: Provide sqlite backend to skip mysql installation later
-  apt:
-    name:
-      - akonadi-backend-sqlite
-    state: latest
-  environment:
-    http_proxy: http://firewall.pn.steinbeis.schule:3128
-    https_proxy: http://firewall.pn.steinbeis.schule:3128
-
 - name: Install desktop EDU packages and some more
   apt:
     name:
@@ -17,11 +8,12 @@
       - xdg-desktop-portal-kde
       - xdg-desktop-portal-wlr # share screen in browser
       - kde-full
-      - atftp
+      - akonadi-backend-sqlite
       - thunderbird-l10n-de
       - webext-privacy-badger
       - webext-ublock-origin-firefox
       - webext-ublock-origin-chromium
+      - atftp
       - emacs
       - elpa-magit
       - elpa-color-theme-modern