For some reason you may have damaged an archive and/or lock file. You may try to remove the file /var/cache/apt/pkgcache.bin and recreate it with
Code:
sudo rm -r /var/cache/apt/pkgcache.bin
sudo apt-get keep-all
You may also look at the /var/lib/dpkg directory, that may be compromised as well.
NOTE!! I never tried this, so it's really on your own risk. As an alternative I suggest re-installing Ubuntu.