We have the same problem on our project.
We have diagnosed the problem. 
Our problem was traced back to a 3rd party Revit Add-in, in this case it was the Easy Keynoter Add-in. 
The quick fix is to move the Add-ins .addin launcher file into a "disabled" folder within the Addins directory. 
We will have to explore this issue further with the App developer and update this forum thread with feedback.