I'm trying to upgrade my Debian Lenny to Squeeze. I've replaced the word lenny to squeeze in sources.list and ran

apt-get clean
apt-get update
apt-get dist-upgrade

But after a while, I get this error

Preconfiguring packages ...
Setting up debian-archive-keyring (2010.08.28) ...
ERROR: Can't find the archive-keyring
Is the ubuntu-keyring package installed?
dpkg: error processing debian-archive-keyring (--configure):
 subprocess installed post-installation script returned error exit status 1
Errors were encountered while processing:
 debian-archive-keyring
E: Sub-process /usr/bin/dpkg returned an error code (1)

So I tried to install apt-get -f install debian-archive-keyring and I got the same error. Then I tried to install apt-get -f install ubuntu-keyring and I got this error:

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package ubuntu-keyring is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package ubuntu-keyring has no installation candidate

Maybe I have the wrong sources in my sources.list:

deb    ftp://mirror.hetzner.de/debian/packages  squeeze          main contrib non-free
deb    ftp://mirror.hetzner.de/debian/security  squeeze/updates  main contrib non-free

deb     http://ftp.de.debian.org/debian/  squeeze  main non-free contrib
deb-src http://ftp.de.debian.org/debian/  squeeze  main non-free contrib

deb     http://security.debian.org/  squeeze/updates  main contrib non-free
deb-src http://security.debian.org/  squeeze/updates  main contrib non-free

Hope anyone can help me,

thx, tux