to follow up on my own mail, someone just pointed out to me that I should set up the file '/etc/apt/sources.list' followed by 'apt-get update' etc. This seems to solve all issues, at least now I'm happily continuing the installation. To whomever it concerns: this should be added to the documentation at http://www.debian.org/releases/stable/i386/ch-preparing.en.html#s-linux-upgrade ! Regards, Stefan