Upgraded Web application to .NET 4 have problems communicating with remoting server

Question

I recently upgraded an application that was .NET 3 to .NET 4. I did this automatically by loading it into Visual Studio 2012 which did the upgrade. This application communicates with another server via remoting. The application works perfectly fine on the old version, but after upgrading it I get the following error:

[NullReferenceException: Object reference not set to an instance of an object.]

I believe this is happening because of the following :

tempObj = Activator.GetObject(GetType(Tech.Service.Interfaces.IGetTechRates), ConfigurationManager.AppSettings("TECHser"))

The null reference is happening at the following line:

Dim TechDV As DataView = New DataView(ods.Tables(0), _
                                  sRowFilter, _
                                  sSortExpression, _
                                  DataViewRowState.CurrentRows)

Where ods is:

ods = tempObj.GetTechList

I have both versions of the application, and I have been switching between the two of them. The .NET 3 version works fine but the .NET 4 version is giving problems.

This is not a question about what NullReferenceException is. I am asking what happened during the upgrade that could have caused this, since Visual Studio said that everything should work as before.