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sascha.heil
Can confirm this behaviour. Removing and reinserting a widget works (for me).
This problem is caused by an incorrect report URL being stored for the widget. Removing/reinserting a widget will fix it (as you've discovered). You can also change the view (ie, if it's a bar graph, click on the pie graph icon).
No idea what caused incorrect URLs to be stored, though. My best guess would be that its due to updating Piwik from some specific version.