Comment 14 for bug 1610500

Christian, my reproduction steps were wrong, "do-release-upgrade --proposed" in a Trusty container won't work because this will only pull the upgrader from xenial-proposed and not pptpd. As such, there is no easy way to do a test release upgrade but you can fix a broken upgrade by installing 1.4.0-7ubuntu0.2 from xenial-proposed. Lastly, the unauthenticated warning from your Trusty container seems to be unrelated, try refreshing your indexes (apt-get update) and see if that helps.