Comment 3 for bug 1571174

I tried the same again in a Docker container:

1.) docker run --privileged -ti ubuntu:15.10 /bin/bash
2.) apt-get update; apt-get -y dist-upgrade
3.) apt-get install ufw squid-deb-proxy wget
4.) service squid-deb-proxy start
5.) wget http://archive.ubuntu.com/ubuntu/pool/main/u/ufw/ufw_0.35-0ubuntu2_all.deb
6.) dpkg -i ufw_0.35-0ubuntu2_all.deb
7.) due to https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+bug/1570947 did
export SUDO_USER=root
export SUDO_UID=0
export SUDO_COMMAND=/bin/bash
export SUDO_GID=0
8.) do-release-upgrade -d

completed without further errors. --> upgrading ufw before squid helps