Software Center вылетает, и я не могу больше устанавливать пакеты

После того, как я попытался установить Skype (для этого я набрал несколько команд, которые не помню, что получил с этого форума), мой центр программного обеспечения закрывается сразу после его открытия, и я не могу установить какие-либо пакеты через терминал больше.

Также я получаю это сообщение об ошибке в строке меню:

An error occurred. Please run package manager from the right-click menu or apt-get
in a terminal to see what is wrong. The error message was: Unknown error '<class<
'SystemError'>' (E:Malformed line 56 in source list (dist parse))'. This usually
means that your installed packages have unmet dependencies

И это то, что я получаю в терминале, когда я даю команду software-center:

derya1991@derya1991-Lenovo-IdeaPad-U450:~$ software-center
2015-03-19 23:24:00,957 - softwarecenter.ui.gtk3.app - INFO - setting up proxy 'None'
2015-03-19 23:24:04,466 - softwarecenter.region - WARNING - failed to use geoclue: 'org.freedesktop.Geoclue.Error.notAvailable: Geoclue master client has no usable Address providers'
2015-03-19 23:24:05,411 - softwarecenter.backend.reviews - WARNING - Could not get usefulness from server, no username in config file
2015-03-19 23:24:05,437 - softwarecenter.plugin - INFO - activating plugin '<module 'webapps_activation' from '/usr/share/software-center/softwarecenter/plugins/webapps_activation.pyc'>'
2015-03-19 23:24:05,588 - softwarecenter.db.pkginfo_impl.aptcache - INFO - aptcache.open()
2015-03-19 23:24:06,758 - softwarecenter.backend.reviews - WARNING - error creating bsddb: '(22, 'Invalid argument -- BDB0054 illegal flag combination specified to DB_ENV->open')' (corrupted?)
2015-03-19 23:24:06,773 - softwarecenter.backend.reviews - ERROR - trying to repair DB failed
Traceback (most recent call last):
  File "/usr/share/software-center/softwarecenter/backend/reviews/__init__.py", line 358, in _save_review_stats_cache_blocking
    self._dump_bsddbm_for_unity(outfile, outdir)
  File "/usr/share/software-center/softwarecenter/backend/reviews/__init__.py", line 377, in _dump_bsddbm_for_unity
    0600)
DBInvalidArgError: (22, 'Invalid argument -- BDB0054 illegal flag combination specified to DB_ENV->open')
/usr/lib/python2.7/dist-packages/gi/overrides/GLib.py:535: Warning: Source ID 76 was not found when attempting to remove it
  return super(MainContext, self).iteration(may_block)
Traceback (most recent call last):
  File "/usr/share/software-center/softwarecenter/db/pkginfo_impl/aptcache.py", line 261, in open
    self._cache = apt.Cache(progress)
  File "/usr/lib/python2.7/dist-packages/apt/cache.py", line 107, in __init__
    self.open(progress)
  File "/usr/lib/python2.7/dist-packages/apt/cache.py", line 155, in open
    self._list.read_main_list()
SystemError: E:Malformed line 56 in source list /etc/apt/sources.list (dist parse)
2015-03-19 23:24:11,008 - softwarecenter.db.enquire - ERROR - _get_estimate_nr_apps_and_nr_pkgs failed
Traceback (most recent call last):
  File "/usr/share/software-center/softwarecenter/db/enquire.py", line 115, in _get_estimate_nr_apps_and_nr_pkgs
    tmp_matches = enquire.get_mset(0, len(self.db), None, xfilter)
  File "/usr/share/software-center/softwarecenter/db/appfilter.py", line 89, in __call__
    if (not pkgname in self.cache and
  File "/usr/share/software-center/softwarecenter/db/pkginfo_impl/aptcache.py", line 281, in __contains__
    return self._cache.__contains__(k)
AttributeError: 'NoneType' object has no attribute '__contains__'
Traceback (most recent call last):
  File "/usr/bin/software-center", line 183, in <module>
    app.run(args)
  File "/usr/share/software-center/softwarecenter/ui/gtk3/app.py", line 1378, in run
    self.show_available_packages(args)
  File "/usr/share/software-center/softwarecenter/ui/gtk3/app.py", line 1316, in show_available_packages
    self.view_manager.set_active_view(ViewPages.AVAILABLE)
  File "/usr/share/software-center/softwarecenter/ui/gtk3/session/viewmanager.py", line 150, in set_active_view
    view_widget.init_view()
  File "/usr/share/software-center/softwarecenter/ui/gtk3/panes/availablepane.py", line 227, in init_view
    self.cache, self.db, self.icons, self.apps_filter)
  File "/usr/share/software-center/softwarecenter/ui/gtk3/views/lobbyview.py", line 80, in __init__
    self.build()
  File "/usr/share/software-center/softwarecenter/ui/gtk3/views/lobbyview.py", line 326, in build
    self._build_homepage_view()
  File "/usr/share/software-center/softwarecenter/ui/gtk3/views/lobbyview.py", line 121, in _build_homepage_view
    self._append_whats_new()
  File "/usr/share/software-center/softwarecenter/ui/gtk3/views/lobbyview.py", line 255, in _append_whats_new
    whats_new_cat = self._update_whats_new_content()
  File "/usr/share/software-center/softwarecenter/ui/gtk3/views/lobbyview.py", line 240, in _update_whats_new_content
    docs = whats_new_cat.get_documents(self.db)
  File "/usr/share/software-center/softwarecenter/db/categories.py", line 131, in get_documents
    nonblocking_load=False)
  File "/usr/share/software-center/softwarecenter/db/enquire.py", line 330, in set_query
    self._blocking_perform_search()
  File "/usr/share/software-center/softwarecenter/db/enquire.py", line 225, in _blocking_perform_search
    matches = enquire.get_mset(0, self.limit, None, xfilter)
  File "/usr/share/software-center/softwarecenter/db/appfilter.py", line 89, in __call__
    if (not pkgname in self.cache and
  File "/usr/share/software-center/softwarecenter/db/pkginfo_impl/aptcache.py", line 281, in __contains__
    return self._cache.__contains__(k)
AttributeError: 'NoneType' object has no attribute '__contains__'

Что я могу сделать, чтобы решить эту проблему? У меня последняя версия Ubuntu, и я абсолютный новичок.