package squid 3.5.12-1ubuntu7 failed to install/upgrade: subprocess installed post-installation script returned error exit status 1

[Brandon Thetford]

do-release-upgrade
From 15.10 to 16.04

ProblemType: Package
DistroRelease: Ubuntu 16.04
Package: squid 3.5.12-1ubuntu7
Uname: Linux 4.5.2-040502-generic x86_64
ApportVersion: 2.19.1-0ubuntu5
Architecture: amd64
Date: Fri Apr 29 02:15:20 2016
DuplicateSignature: package:squid:3.5.12-1ubuntu7:subprocess installed post-installation script returned error exit status 1
ErrorMessage: subprocess installed post-installation script returned error exit status 1
InstallationDate: Installed on 2015-09-22 (219 days ago)
InstallationMedia: Ubuntu-Server 15.04 "Vivid Vervet" - Release amd64 (20150422)
RelatedPackageVersions:
 dpkg 1.18.4ubuntu1
 apt 1.2.10ubuntu1
SourcePackage: squid3
Title: package squid 3.5.12-1ubuntu7 failed to install/upgrade: subprocess installed post-installation script returned error exit status 1
UpgradeStatus: Upgraded to xenial on 2016-04-29 (0 days ago)
mtime.conffile..etc.squid.squid.conf: 2016-04-22T04:55:53.994525

[Brandon Thetford (dodexahedron)]

This causes all package upgrades after squid3 to fail and the upgrade to be reported as successful with errors.
The system now boots up as 16.04, but the rest of the packages still had to be installed to "truly" finish the release upgrade.

This is easy enough to resolve by uninstalling the squid3 package and installing the squid package, since 16.04 has a newer version than 15.10 did anyway.
Just be sure you don't lose your configuration (make a backup).

apt-get update
apt-get remove squid3
apt-get install squid
apt-get upgrade

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.