Not able to upgrade any .deb package

[Davide86]

Distribution: Ubuntu 11.04 natty
Application: Ubuntu Tweak 0.5.13
Desktop: gnome

Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/ubuntutweak/mainwindow.py", line 502, in setup_notebook
    page = module()
  File "/usr/lib/python2.7/dist-packages/ubuntutweak/modules/updatemanager.py", line 44, in __init__
    self.update_list()
  File "/usr/lib/python2.7/dist-packages/ubuntutweak/modules/updatemanager.py", line 59, in update_list
    PACKAGE_WORKER.update_apt_cache(init=True)
  File "/usr/lib/python2.7/dist-packages/ubuntutweak/common/package.py", line 226, in update_apt_cache
    self.cache = apt.Cache()
  File "/usr/lib/python2.7/dist-packages/apt/cache.py", line 89, in __init__
    self.open(progress)
  File "/usr/lib/python2.7/dist-packages/apt/cache.py", line 127, in open
    self._cache = apt_pkg.Cache(progress)
SystemError: E:Encountered a section with no Package: header, E:Problem with MergeList /var/lib/apt/lists/it.archive.ubuntu.com_ubuntu_dists_natty-updates_main_i18n_Translation-it, E:L'elenco dei pacchetti o il file di stato non può essere letto o aperto.
"the list of packages or state files cannot be read or open"

[Davide86 (luciferoserra)]

This error do not depend on "ubuntu tweak"... Also the package manager and ubuntu software center return the same error.