Error calling .Net lib from VB script, calling from VB6 executable works

When I create a C# .Net library (Framework 2.0 and Visual Studio 2005) and expose it to com, it works great from a Visual Basic 6 client but doesn't work from a VB script client (such as Excel).

The error message from Microsoft Visual Basic in Excel is:
Run-time error '-2147024894(800070002)';
File or assembly name TestCallingDotNetFromCom, or one of its dependencies, was not found.

Here's the C# library code:
using System;
using System.Collections.Generic;
using System.Text;
using System.Runtime.InteropServices;

namespace TestCallingDotNetFromCom
{
    public interface iTestStuff
    {
        string SayHello();
    }

    [ClassInterface(ClassInterfaceType.None)]
    public class TestStuff : iTestStuff
    {
        public TestStuff()
        {
        }

        public string SayHello()
        {
            return "Hello there";
        }
    }
}

It's complied as a strong-name assembly with the Make assembly COM-Visible checkbox checked. I copy the library to another machine (XP Professional) and register it as:
regasm TestCallingDotNetFromCom.dll /tlb
This creates the type library and registers both the type library and the library. Then I copy the library to the GAC.

I open up the Visual Basic editor in Excel (XP version) and set a reference to TestCallingDotNetFromCom. Examining it from the Object Browser, it looks fine. I add a module with the code:
Sub Test()
    Dim obj As New TestCallingDotNetFromCom.TestStuff, _
        s As String
    s = obj.SayHello
    Set obj = Nothing
End Sub
When I run the code I get the error mentioned above.

HOWEVER, I use Visual Basic 6 to create an executable with EXACTLY  the same code, then copy the exe to the test machine and run it, it works perfectly!

What's the difference between calling a com callable .Net library from Visual Basic 6 and Visual Basic script in Excel?

Appreciate any help!