COMInterop - System cannot find file - IDE only

I've an old VB6 program which I have haven't used for several months. The program references numerous C# assemblies (.NetFramework 4.8). Running the programs as binaries, it all works fine.

I have opened the VB6 ide to step some of my code and I am now encountering an automation error "The system cannot find the file specified." (Err 80070002) when trying to instantiate one of the COM Interop classes. This didn't used to happen.

Private Sub InitMessageStore()

    Dim l_oBusFactory As IfxBusService.BusFactory <= COM Interop reference

    Set l_oBusFactory = New IfxBusService.BusFactory <= ERROR here
    l_oBusFactory.InitialiseMessageStore GetConnection(m_oIfxsys.Dbase.Database.Definition)

End Sub

The COM Interop decls

[ComVisible(true)]
[ClassInterface(ClassInterfaceType.None)]
[ComDefaultInterface(typeof(IBusFactory))]
[Guid("200C6C26-6881-4CB5-A8E7-E0E5532D6D5F")]
public class BusFactory : IBusFactory


[ComVisible(true)]
[InterfaceType(ComInterfaceType.InterfaceIsIDispatch)]
[Guid("5531BD20-2C7B-452B-A7D7-2D05E39EB83E")]
public interface IBusFactory

I have rechecked the registry registrations for the above ... the salient one being

and the file is in the folder location as specified in the registry.

I'm using Win10 Version 10.0.19044 Build 19044. I'm running everything as administrator. I have DEP switched to essential Windows programs and services only.

As I said above, everything works fine when run as binaries, but, to me, it looks like the OS is stopping the VB6 ide from loading a COM Interop assembly.

Any suggestions as to how I can get stepping through (but not necessarily into) my COM Interop code working again.