qtile: Notifications widget does not work

Notification widget does not show notifications. Log file is filled with the following messages, which started to appear 2 weeks ago:

2016-04-13 16:13:23,852 ERROR libqtile notify.py:service():L94 Dbus connection failed
Traceback (most recent call last):
  File "/usr/lib/python3.5/site-packages/qtile-0.10.5-py3.5.egg/libqtile/notify.py", line 92, in service
    self._service = NotificationService(self)
  File "/usr/lib/python3.5/site-packages/qtile-0.10.5-py3.5.egg/libqtile/notify.py", line 45, in __init__
    service.Object.__init__(self, bus_name, SERVICE_PATH)
  File "/usr/lib/python3.5/site-packages/dbus/service.py", line 485, in __init__
    self.add_to_connection(conn, object_path)
  File "/usr/lib/python3.5/site-packages/dbus/service.py", line 576, in add_to_connection
    self._fallback)
RuntimeError: To make asynchronous calls, receive signals or export objects, D-Bus connections must be attached to a main loop by passing mainloop=... to the constructor or calling dbus.set_default_main_loop(...)
2016-04-13 16:13:23,853 WARNING libqtile notify.py:register():L102 Registering update without any dbus connection existing

Best regards, Toller