What does this mean:

Error: System.InvalidCastException: Unable to cast COM object of type 'Microsoft.Office.Interop.Excel.ApplicationClass' to interface type 'Microsoft.Office.Interop.Excel._Application'.  

In short this is a Microsoft issue, and has been since I started Bonus Tools back in 2010, and will effect other Revit addins like Dynamo.

The reason behind the error is, when Microsoft does their updates, they do not do all the clean-up they should. 

Performing a Office repair resolves the issue on my PC every time it happens.