ubuntu software center does not open at all

/usr/bin/software-center:25: PyGIWarning: Gtk was imported without specifying a version first. Use gi.require_version('Gtk', '3.0') before import to ensure that the right version gets loaded.
  from gi.repository import Gtk, GObject
/usr/share/software-center/softwarecenter/ui/gtk3/views/purchaseview.py:29: PyGIWarning: WebKit2 was imported without specifying a version first. Use gi.require_version('WebKit2', '4.0') before import to ensure that the right version gets loaded.
  from gi.repository import WebKit2 as webkit
2016-08-09 01:42:45,103 - softwarecenter.backend.zeitgeist_logger - WARNING - Support for Zeitgeist disabled
/usr/share/software-center/softwarecenter/ui/gtk3/widgets/symbolic_icons.py:23: PyGIWarning: PangoCairo was imported without specifying a version first. Use gi.require_version('PangoCairo', '1.0') before import to ensure that the right version gets loaded.
  from gi.repository import Gtk, Gdk, GLib, PangoCairo
2016-08-09 01:42:45,131 - softwarecenter.ui.gtk3.app - INFO - setting up proxy 'None'
2016-08-09 01:42:45,224 - softwarecenter.fixme - WARNING - logs to the root logger: '('/usr/lib/python2.7/dist-packages/dbus/proxies.py', 410, '_introspect_error_handler')'
2016-08-09 01:42:45,224 - dbus.proxies - ERROR - Introspect error on com.ubuntu.sso:/com/ubuntu/sso/credentials: dbus.exceptions.DBusException: org.freedesktop.DBus.Error.Spawn.ChildExited: Process com.ubuntu.sso exited with status 1
Traceback (most recent call last):
  File "/usr/bin/software-center", line 130, in <module>
    app = SoftwareCenterAppGtk3(options, args)
  File "/usr/share/software-center/softwarecenter/ui/gtk3/app.py", line 338, in __init__
    self.icons)
  File "/usr/share/software-center/softwarecenter/ui/gtk3/session/appmanager.py", line 66, in __init__
    self.oauth_token = helper.find_oauth_token_sync()
  File "/usr/share/software-center/softwarecenter/backend/ubuntusso.py", line 143, in find_oauth_token_sync
    sso.find_credentials()
  File "/usr/share/software-center/softwarecenter/backend/login_impl/login_sso.py", line 74, in find_credentials
    self.proxy.find_credentials(self.appname, self._get_params())
  File "/usr/lib/python2.7/dist-packages/dbus/proxies.py", line 70, in __call__
    return self._proxy_method(*args, **keywords)
  File "/usr/lib/python2.7/dist-packages/dbus/proxies.py", line 145, in __call__
    **keywords)
  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: Process com.ubuntu.sso exited with status 1

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: software-center 16.01+16.04.20160420
ProcVersionSignature: Ubuntu 4.4.0-34.53-generic 4.4.15
Uname: Linux 4.4.0-34-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
CurrentDesktop: Unity
Date: Tue Aug 9 01:35:00 2016
InstallationDate: Installed on 2016-03-11 (151 days ago)
InstallationMedia: Ubuntu 15.10 "Wily Werewolf" - Release amd64 (20151021)
PackageArchitecture: all
ProcEnviron:
 LANGUAGE=es_AR:es
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=es_AR.UTF-8
 SHELL=/bin/bash
SourcePackage: software-center
UpgradeStatus: Upgraded to xenial on 2016-04-28 (102 days ago)