Fixes #8014 the Diagnostics plugin need to always be enabled

core/Plugin/Manager.php

@@ -65,6 +65,7 @@ class Manager
     // These are always activated and cannot be deactivated
     protected $pluginToAlwaysActivate = array(
         'CoreHome',
+        'Diagnostics',
         'CoreUpdater',
         'CoreAdminHome',
         'CoreConsole',