It was a bit longer so I try to post here.
Maybe "apt-get -f install" to force an install of the files that didn't get loaded because of the error. Then try "apt-get upgrade" again, "apt-get -f install" back and forth until only the package that has the error is left.
Or try "cd /etc/alternatives/" and "ls -l desktop-grub"
Should give you something like lrwxrwxrwx 1 root root 54 Mär 10 17:59 desktop-grub -> /usr/share/desktop-base/active-theme/grub/grub-4x3.png .
Maybe some links are broken and you could try to fix it with yes '' | update-alternatives --force --all