While I can not answer why it happened, this is what I did to get it working after upgrading from 15.10 server to 16.04 server.
Removing apt-cacher-ng and reinstalling did not solve the problem, but purging apt-cacher-ng and then reinstalling works. Confirmed on two machines. Hope this helps someone. ** Description changed: I don't know what happened. ProblemType: Package DistroRelease: Ubuntu 16.04 Package: apt-cacher-ng 0.9.1-1ubuntu1 ProcVersionSignature: Ubuntu 4.4.0-23.41-generic 4.4.10 Uname: Linux 4.4.0-23-generic x86_64 NonfreeKernelModules: wl ApportVersion: 2.20.1-0ubuntu2.1 Architecture: amd64 Date: Mon May 23 01:30:10 2016 ErrorMessage: subprocess installed post-installation script returned error exit status 1 InstallationDate: Installed on 2016-05-21 (2 days ago) InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1) RelatedPackageVersions: - dpkg 1.18.4ubuntu1.1 - apt 1.2.12~ubuntu16.04.1 + dpkg 1.18.4ubuntu1.1 + apt 1.2.12~ubuntu16.04.1 SourcePackage: apt-cacher-ng Title: package apt-cacher-ng 0.9.1-1ubuntu1 failed to install/upgrade: subprocess installed post-installation script returned error exit status 1 UpgradeStatus: No upgrade log present (probably fresh install) -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1584816 Title: package apt-cacher-ng 0.9.1-1ubuntu1 failed to install/upgrade: subprocess installed post-installation script returned error exit status 1 To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/apt-cacher-ng/+bug/1584816/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs