Problem with Ubuntu Software Center--can't install anything

Problem with Ubuntu Software Center--can't install anything

About 3 weeks ago I downloaded Ubuntu 12.04 and I've been running it since
then. Initially everything worked great, but about 10 days ago the Ubuntu
Software Center quit working (I previously used it with no problems).Now,
I can open it up and find programs, but when I click the Install button
for one of them, nothing happens. This issue seems to be specific to the
Software Center--I can still use apt-get from the command line with no
problems.
I've tried removing and reinstalling the Software Center, and I've tried
running it from the terminal using sudo software-center (in case there was
some issue with permissions). When I use the terminal, upon starting up
software-center a long error message is printed to the terminal (yes, an
apparently identical traceback is printed twice):
2013-08-25 12:38:50,829 - softwarecenter.ui.gtk3.app - INFO - setting up
proxy 'None'
2013-08-25 12:38:50,833 - softwarecenter.db.database - INFO - open()
database: path=None use_axi=True use_agent=True
2013-08-25 12:38:51,079 - softwarecenter.backend.reviews - WARNING - Could
not get usefulness from server, no username in config file
2013-08-25 12:38:51,178 - softwarecenter.ui.gtk3.app - INFO -
show_available_packages: search_text is '', app is None.
2013-08-25 12:38:51,181 - softwarecenter.db.pkginfo_impl.aptcache - INFO -
aptcache.open()
Traceback (most recent call last):
File "/usr/lib/python2.7/dist-packages/dbus/connection.py", line 607, in
msg_reply_handler
*message.get_args_list()))
File "/usr/lib/python2.7/dist-packages/dbus/bus.py", line 69, in error_cb
callback('')
File
"/usr/share/software-center/softwarecenter/backend/installbackend_impl/aptd.py",
line 153, in _register_active_transactions_watch
apt_daemon = client.get_aptdaemon(bus=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/dbus/connection.py", line 607, in
msg_reply_handler
*message.get_args_list()))
File "/usr/lib/python2.7/dist-packages/dbus/bus.py", line 69, in error_cb
callback('')
File
"/usr/share/software-center/softwarecenter/backend/installbackend_impl/aptd.py",
line 153, in _register_active_transactions_watch
apt_daemon = client.get_aptdaemon(bus=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
2013-08-25 12:38:53,215 - softwarecenter.ui.gtk3.widgets.exhibits -
WARNING - download failed: '<class 'gi._glib.GError'>', 'Operation not
supported'
Any suggestions?