Run-time error 429 in VBA with C# COM Addin

I made a COM addin for Excel VBA last year. It worked fine with Excel 2010 under Windows 7. Recently, I upgraded to Office 2013 and Windows 8. This addin never worked anymore. I am given a message that says "Run-time error 429. ActiveX component can't create object." 

I can see function definitions in Object Browers in VBA. Everything looks fine over there. And the VBA code that instantiates the C# made COM class looks ok as well. I am suspecting either the registration procedure is not compatible or some system settings need to set straight. Please help.

By the way, the C# project's target framework is .net 3.5. I did not change a thing about it. The new system has the framework installed (and rebooted). Also, I made a test project in C# to see if there's some coding errors in the Addin project. The functionality of the Addin works just as expected in test project.

Here's the C# end of code that registers the COM add-in:

using System;
using System.Collections.Generic;
using System.Runtime.InteropServices;
using Microsoft.Win32;

namespace BlahBlahBlah
{
    [Guid("ABCDEFT - I marked off real GUID")]
    [ComDefaultInterface(typeof(iFMFunctions))]
    [ComVisible(true)]
    //[ClassInterface(ClassInterfaceType.AutoDual)]
    [ComSourceInterfaces(typeof(iFMFunctionsEvent))]
    public partial class Functions : iFMFunctions
    {

        [ComRegisterFunctionAttribute]
        public static void RegisterFunction(Type type)
        {           
            RegistryKey InrpoSrv32Key = null;
            InrpoSrv32Key = Registry.ClassesRoot.OpenSubKey("CLSID\\{" + type.GUID.ToString().ToUpper() + "}\\InprocServer32", true);
            InrpoSrv32Key.SetValue("", System.Environment.SystemDirectory + @"\mscoree.dll");
            InrpoSrv32Key.Close();

            Registry.ClassesRoot.OpenSubKey("CLSID\\{" + type.GUID.ToString().ToUpper() + "}\\Programmable", true);
        }

        [ComUnregisterFunctionAttribute]
        public static void UnregisterFunction(Type type)
        {
            try
            {
                Registry.ClassesRoot.DeleteSubKey("CLSID\\{" + type.GUID.ToString().ToUpper() + "}\\InprocServer32", false);
                Registry.ClassesRoot.DeleteSubKey("CLSID\\{" + type.GUID.ToString().ToUpper() + "}\\Programmable", false);
            }
            catch { }
        }
    }

}

Here's VBA end of the code:

    Dim aa As New BlahBlahBlah.Functions
    Dim d As Double
    'd = aa.SOMEREALFUNCTION(100, 100, 0.09, 0.2, 1, True)