How can I fix Update Manager in 12.04, which hangs when I try to install any update, while sudo apt-get upgrade works fine? I suspect it has something to do with my python installation. This is the error message:

Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/defer/__init__.py", line 475, in _inline_callbacks
    result = gen.send(result)
  File "/usr/lib/python2.7/dist-packages/aptdaemon/client.py", line 1622, in _run_transaction_helper
    daemon = get_aptdaemon(self.bus)
  File "/usr/lib/python2.7/dist-packages/aptdaemon/client.py", line 1696, in get_aptdaemon
    False),
  File "/usr/lib/python2.7/dist-packages/dbus/bus.py", line 241, in get_object
    follow_name_owner_changes=follow_name_owner_changes)
  File "/usr/lib/python2.7/dist-packages/dbus/proxies.py", line 248, in __init__
    self._named_service = conn.activate_name_owner(bus_name)
  File "/usr/lib/python2.7/dist-packages/dbus/bus.py", line 180, in activate_name_owner
    self.start_service_by_name(bus_name)
  File "/usr/lib/python2.7/dist-packages/dbus/bus.py", line 278, in start_service_by_name
    'su', (bus_name, flags)))
  File "/usr/lib/python2.7/dist-packages/dbus/connection.py", line 651, in call_blocking
    message, timeout)
dbus.exceptions.DBusException: org.freedesktop.DBus.Error.Spawn.ChildExited: Launch helper exited with unknown return code 1
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/defer/__init__.py", line 475, in _inline_callbacks
    result = gen.send(result)
  File "/usr/lib/python2.7/dist-packages/aptdaemon/client.py", line 1622, in _run_transaction_helper
    daemon = get_aptdaemon(self.bus)
  File "/usr/lib/python2.7/dist-packages/aptdaemon/client.py", line 1696, in get_aptdaemon
    False),
  File "/usr/lib/python2.7/dist-packages/dbus/bus.py", line 241, in get_object
    follow_name_owner_changes=follow_name_owner_changes)
  File "/usr/lib/python2.7/dist-packages/dbus/proxies.py", line 248, in __init__
    self._named_service = conn.activate_name_owner(bus_name)
  File "/usr/lib/python2.7/dist-packages/dbus/bus.py", line 180, in activate_name_owner
    self.start_service_by_name(bus_name)
  File "/usr/lib/python2.7/dist-packages/dbus/bus.py", line 278, in start_service_by_name
    'su', (bus_name, flags)))
  File "/usr/lib/python2.7/dist-packages/dbus/connection.py", line 651, in call_blocking
    message, timeout)
dbus.exceptions.DBusException: org.freedesktop.DBus.Error.Spawn.ChildExited: Launch helper exited with unknown return code 1
dbus.exceptions.DBusException: org.freedesktop.DBus.Error.Spawn.ChildExited: Launch helper exited with unknown return code 1
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/defer/__init__.py", line 473, in _inline_callbacks
    result = gen.throw(result.type, result.value, result.traceback)
  File "/usr/lib/python2.7/dist-packages/UpdateManager/backend/InstallBackendAptdaemon.py", line 52, in commit
    downgrade, defer=True)
dbus.exceptions.DBusException: org.freedesktop.DBus.Error.Spawn.ChildExited: Launch helper exited with unknown return code 1