We fixed it by starting a session of Revit double clicking the file in it's folder vs. opening the file from within Revit.
We do a bit of programming here with Addins and we think the addin has caused the issue. The addin won't load if you start Revit by doublle clicking the filename.
Opening the Revit file with a dobbleclick does the trick for me... Strange! Maybe someone from Autodesk would like to comment the subject?
I came through the same problem after installing the Microsoft .NET Framework 4.5 with Visual Studio 2012. After removing it and reinstaling the previous version of the .NET framework (Version 4), everything went fine.
There is finally a fix for the issue.