Comment 10 for bug 1452968

I experienced the same problem, I think it has been invoked by not first updating + upgrading + dist-upgrading to the latest version of 14.04 (.3 at the time of writing) before running do-release-upgrade

Besides resetting the /etc/apt/sources.list file to trusty, as suggested earlier, "pinning" to utopic sources as added by the failed do-release-upgrade has to be removed from the /etc/apt/preferences file. As also discussed in https://askubuntu.com/questions/615090/broken-upgrade-from-14-04-to-14-10/665663#665663 , I succeeded to fix it for 3 machines that all experienced this problem as follows:

sudo -s
cd /etc/apt

#Remove potential pinning preferences
mv preferences preferences_old

#Restore sources
mv sources.list sources.list.old
cp sources.list.save sources.list

#Clean apt cache
apt-get clean

#Possibly repeat this block until `lsb_release -a` shows 14.04.3 and no further update/dist-upgrades possible
apt-get update
apt-get upgrade
apt-get dist-upgrade

#Finally perform the release upgrade and reboot
do-release-upgrade