Comment 7 for bug 1298793

I ran into this very same problem, and started Googling. Saw that the first time advised was to --purge and/ or --reinstall software-center. I did that too, but it gave me no results. Tried deleting the /home/<user_name>/.cache/software-center as suggested elsewhere. This too had no result.

In the end however, I got software-center working again, due to the remark of another user I read in one of the forums...

(S)he pointed out that, after attempting to start software-center in Terminal, gave:

dbus.exceptions.DBusException: org.freedesktop.DBus.Error.Spawn.ChildExited: Process /usr/lib/ubuntu-sso-client/ubuntu-sso-login exited with status 1

The suggestion was to run in Terminal:
gothqueen@pussycomputer:~$ /usr/lib/ubuntu-sso-client/ubuntu-sso-login

This returned the traceback:
Traceback (most recent call last):
  File "/usr/lib/ubuntu-sso-client/ubuntu-sso-login", line 67, in <module>
    from ubuntu_sso.main import main
  File "/usr/lib/python2.7/dist-packages/ubuntu-sso-client/ubuntu_sso/main/__init__.py", line 41, in <module>
    from ubuntu_sso import utils
  File "/usr/lib/python2.7/dist-packages/ubuntu-sso-client/ubuntu_sso/utils/__init__.py", line 46, in <module>
    from ubuntu_sso.logger import setup_logging
  File "/usr/lib/python2.7/dist-packages/ubuntu-sso-client/ubuntu_sso/logger.py", line 61, in <module>
    backupCount=5)
  File "/usr/lib/python2.7/logging/handlers.py", line 117, in __init__
    BaseRotatingHandler.__init__(self, filename, mode, encoding, delay)
  File "/usr/lib/python2.7/logging/handlers.py", line 64, in __init__
    logging.FileHandler.__init__(self, filename, mode, encoding, delay)
  File "/usr/lib/python2.7/logging/__init__.py", line 903, in __init__
    StreamHandler.__init__(self, self._open())
  File "/usr/lib/python2.7/logging/__init__.py", line 928, in _open
    stream = open(self.baseFilename, self.mode)
IOError: [Errno 13] Permission denied: u'/home/gothqueen/.cache/sso/sso-client.log'

Hence, I looked at the "Permission denied" mentioned; it turned out that the directory /home/gothqueen/.cache/sso/ and the files in it had root permissions. Hence, software center couldn't open them (runs with user permissions), hence permission denied, hence software-center does not open/ start.

My quick solution:
Close software center if open (not likely).
Delete the directory ~/.cache/sso/ completely. Naturally, you'll need root permission to do so.
Open software center.
The ~/.cache/sso/ directory will be created again automagically, but this time with <user> permissions.
Fix done

(PS - Because this worked for me, I didn't try to change the permissions of the original directory/ files.)