There is an error in the German translation file, which causes an error in the upgrade notification message. Attached is a screenshot of the broken upgrade notification and a patch against de.php to fix the problem.
Attachment: Screenshot of the broken message piwik-upgrade.png
Attachment: Patch against lang/de.php piwik-upgrade.patch
(In ) fixes #878 - update German translation (forwarded a copy of the patch to translations (at) piwik.org