iThe following packages have unmet dependencies: icedtea-7-jre-jamvm: Depends: openjdk-7-jre-headless (= 7u75-2.5.4-1~precise1) but 7u55-2.4.7-1ubuntu1~0.12.04.2 is installed Depends: zlib1g (>= 1:1.1.4) but 1:1.2.3.4.dfsg-3ubuntu4 is installed openjdk-7-jre: Depends: openjdk-7-jre-headless (= 7u75-2.5.4-1~precise1) but 7u55-2.4.7-1ubuntu1~0.12.04.2 is installed Depends: libjpeg8 (>= 8c) but 8c-2ubuntu7 is installed

[vinh.]

The package system is broken

Check if you are using third party repositories. If so disable them, since they are a common source of problems.
Furthermore run the following command in a Terminal: apt-get install -f

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.15
ProcVersionSignature: Ubuntu 3.2.0-55.85-generic-pae 3.2.51
Uname: Linux 3.2.0-55-generic-pae i686
NonfreeKernelModules: wl
ApportVersion: 2.0.1-0ubuntu17.6
Architecture: i386
Date: Sat Mar 14 09:11:28 2015
DpkgHistoryLog.txt:

DpkgTerminalLog.txt:

GsettingsChanges:

InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Release i386 (20120423)
MarkForUpload: True
PackageArchitecture: all
ProcEnviron:
 LANGUAGE=en_US:en
 TERM=unknown
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: update-manager
Symptom: ubuntu-release-upgrader-core
UpgradeStatus: Upgraded to precise on 2015-03-14 (0 days ago)

[vinh. (vntour)]

After updating your package information, the essential package 'ubuntu-minimal' could not be located. This may be because you have no official mirrors listed in your software sources, or because of excessive load on the mirror you are using. See /etc/apt/sources.list for the current list of configured software sources.
In the case of an overloaded mirror, you may want to try the upgrade again later.