Login: Detect redirect loop and give users useful advice

src/pretix/control/templates/pretixcontrol/auth/login.html

@@ -19,6 +19,14 @@
             </ul>
             <br>
         {% endif %}
+        {% if possible_cookie_problem %}
+            <div class="alert alert-warning">
+                {% blocktrans trimmed %}
+                    It looks like your browser is not accepting our cookie and you need to log in repeatedly. Please
+                    check if your browser is set to block cookies, or delete all existing cookies and retry.
+                {% endblocktrans %}
+            </div>
+        {% endif %}
         {% csrf_token %}
         {% bootstrap_form form %}
         <div class="form-group buttons">

src/pretix/control/views/auth.py

@@ -149,6 +149,8 @@ def login(request):
             return process_login(request, form.user_cache, form.cleaned_data.get('keep_logged_in', False))
     else:
         form = LoginForm(backend=backend, request=request)
+        # Detect redirection loop (usually means cookie not accepted)
+        ctx['possible_cookie_problem'] = request.path in request.headers.get("Referer", "")
     ctx['form'] = form
     ctx['can_register'] = settings.PRETIX_REGISTRATION
     ctx['can_reset'] = settings.PRETIX_PASSWORD_RESET