Now it works more or less

schoolapps/dashboard/views.py

 from django.contrib.auth.decorators import login_required
 from django.shortcuts import render, redirect
 from django.urls import reverse
+from django.http import HttpResponseNotFound
 from .models import Activity, register_notification
 # from .apps import DashboardConfig
 from mailer import send_mail_with_template
@@ -53,4 +54,4 @@ def test_notification(request):
 
 
 def error_404(request, exception):
-    return render(request, 'common/404.html')
+    return render(request, 'common/404.html')
\ No newline at end of file

schoolapps/schoolapps/settings.py

@@ -10,7 +10,7 @@ from .secure_settings import *
 
 BASE_DIR = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
 
-DEBUG = True
+DEBUG = False
 
 # PDB debugger option
 POST_MORTEM = True

schoolapps/schoolapps/urls.py

@@ -84,4 +84,4 @@ urlpatterns = [
 urlpatterns += static(settings.STATIC_URL, document_root=settings.STATIC_ROOT)
 urlpatterns += static(settings.MEDIA_URL, document_root=settings.MEDIA_ROOT)
 
-# handler404 = 'dashboard.views.error_404'
+handler404 = 'dashboard.views.error_404'

schoolapps/templates/404.html → schoolapps/templates/common/404.html

-{% include 'partials/header.html' %}
+{ include 'partials/header.html' %}
 
 <main>
     <h3>Leider existiert diese Seite nicht. (Fehler 404)</h3>
@@ -11,4 +11,4 @@
 
 </main>
 
-{% include 'partials/footer.html' %}
+{ include 'partials/footer.html' %}