I think I found the bug. If I start the process in a console with "update-notifier --debug-updates" I get this output:
(update-notifier:6904): update-DEBUG: update_check() (update-notifier:6904): update-DEBUG: /usr/lib/update-notifier/apt-check returned 9 (security: 0) (update-notifier:6904): update-DEBUG: error during dbus call: The name com.ubuntu.SystemService was not provided by any .service files (update-notifier:6904): update-DEBUG: interval_days from gconf: 0 It shouldn't search for a **ubuntu** service in **debian**.
