Upgrade from 14.04.2 LTS fails, could not verify libgbm1

[D]

I've tried to install several different version of the library from the ubuntu repositories, but it did not help. I've also tried
apt-key net-update

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ubuntu-release-upgrader-core 1:0.220.7
ProcVersionSignature: Ubuntu 3.19.0-21.21~14.04.1-lowlatency 3.19.8
Uname: Linux 3.19.0-21-lowlatency x86_64
NonfreeKernelModules: fglrx
ApportVersion: 2.14.1-0ubuntu3.11
Architecture: amd64
CrashDB: ubuntu
Date: Wed Jul 1 11:57:34 2015
InstallationDate: Installed on 2013-04-04 (817 days ago)
InstallationMedia: Ubuntu 12.04.2 LTS "Precise Pangolin" - Release amd64 (20130213)
PackageArchitecture: all
ProcEnviron:
 LANGUAGE=de_DE
 TERM=xterm
 PATH=(custom, no user)
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to trusty on 2015-07-01 (0 days ago)
mtime.conffile..etc.update.manager.release.upgrades: 2015-06-29T12:38:04.238953