Install buttons had become inactive

All install buttons in the ubuntu software-center had become inactive. I can select any package but nothing happens when clicking on install. Uninstall is working. I have already completely uninstalled and reinstalled the software-center (using synaptic) but it did not change anything.

---------- terminal output -----------
2013-01-14 19:27:21,598 - softwarecenter.ui.gtk3.app - INFO - setting up proxy 'None'
2013-01-14 19:27:21,611 - softwarecenter.db.database - INFO - open() database: path=None use_axi=True use_agent=True
2013-01-14 19:27:22,616 - softwarecenter.backend.reviews - WARNING - Could not get usefulness from server, no username in config file
2013-01-14 19:27:23,248 - softwarecenter.ui.gtk3.app - INFO - show_available_packages: search_text is '', app is None.
2013-01-14 19:27:24,966 - softwarecenter.db.pkginfo_impl.aptcache - INFO - aptcache.open()
2013-01-14 19:27:32,424 - softwarecenter.db.update - INFO - skipping region restricted app: 'Comentarios Web' (not whitelisted)
2013-01-14 19:27:34,654 - softwarecenter.db.update - INFO - skipping region restricted app: 'reEarCandy' (not whitelisted)
2013-01-14 19:27:35,508 - softwarecenter.db.update - INFO - skipping region restricted app: 'Bulleti d'esquerra de Calonge i Sant Antoni ' (not whitelisted)
2013-01-14 19:27:35,750 - softwarecenter.db.update - INFO - skipping region restricted app: 'Swiftcrypt' (not whitelisted)
2013-01-14 19:27:41,733 - softwarecenter.ui.gtk3.app - INFO - software-center-agent finished with status 0
2013-01-14 19:27:41,734 - softwarecenter.db.database - INFO - reopen() database
2013-01-14 19:27:41,735 - softwarecenter.db.database - INFO - open() database: path=None use_axi=True use_agent=True
------------

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: software-center 5.2.7
ProcVersionSignature: Ubuntu 3.2.0-35.55-generic 3.2.34
Uname: Linux 3.2.0-35-generic x86_64
NonfreeKernelModules: fglrx
ApportVersion: 2.0.1-0ubuntu17.1
Architecture: amd64
Date: Mon Jan 14 19:03:31 2013
EcryptfsInUse: Yes
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Release amd64 (20120425)
MarkForUpload: True
PackageArchitecture: all
ProcEnviron:
 LANGUAGE=de_AT:de
 PATH=(custom, no user)
 LANG=de_AT.UTF-8
 SHELL=/bin/bash
SourcePackage: software-center
UpgradeStatus: No upgrade log present (probably fresh install)