Do you get this error in your ULS-log when visiting a page with a DataViewWebPart that’s been changed via SPD?

Error while executing web part: System.NullReferenceException: Object reference not set to an instance of an object.

Try and uninstall the Update: KB2844286 (Security Update for Microsoft .NET Framewok 3.5.1 on Windows 7 and Windows Server 2008 R2 SP1 for x64-based Systems), which was published 09-07-2013. After removing the update, restart your SharePoint server and everything should be back to normal. Remember to hide the update once it’s uninstalled.

I found the solution over at MSDN thanks to Sergio Orru.

[Update July 6, 2013]:
Looks like Microsoft made a hotfix, to fix KB2844286. It’s KB2872441. Acording to Aaron, that Hotfix will break your workflows though, so beware.