Can't update from Piwik 1.12 (no replies)

Here is a boring but short story about a Piwik that isn't up to date.

I've got a Piwik installation at version 1.12. When I wanted to update it didn't show a new version at the top-right as usual. Even after clicking "Check for updated" it said I'm using the newest version. I then downloaded Piwik 2.2.0 (latest version) to update manually. But I only got a blank page. So I replaced the files again with verson 1.12. Now it works again and after clicking "Check for updates" it now shows "Piwik 2.2.0 is available. Please notify the site administrator."

What can I do?