Comment 1 for bug 156070

try this, first: http://ubuntuforums.org/showthread.php?t=584425
If it doesn't work, here's what I did:

downloaded gutsy.tar.gz.gpg, copied it to tmp, run do-release-upgrade (from a different terminal), copied gutsy.tar.gz.gpg to the tmp directory that update-manager created, commands are:

sudo bash
cd /tmp
wget http://archive.ubuntu.com/ubuntu/dists/gutsy/main/dist-upgrader-all/current/gutsy.tar.gz.gpg
while ! ls | egrep "tmp[^.]" >/dev/null; do echo "waiting..."; done; cp -v gutsy.tar.gz.gpg `ls | egrep "tmp[^.]"`

run do-release-upgrade (from a different terminal)

I think this bug appears because the gutsy.tar.gz.gpg is not copied to tmp directory used by update-manager. I'm not sure why that doesn't happen though.