COM Interop: One interface fails query interface while another works.

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While developing a .NET module that uses COM to communicate with the main application, I have seen this strange error.
I have a class that I expose to COM (BookAndPlanFormSupport) this class implements two interfaces that are also exposed to COM (IDotNetSupportBase and IDotNetMdiSupport).

The resulting IDL looks like this.

coclass BookAndPlanFormSupport {
interface _Object;
interface IDotNetSupportBase;
[default] interface IDotNetMdiSupport;
};

The problem is that in the c++ code query interface on one of the interfaces works, on the other it doesn't.

Example code:
#import "WrapperInterfaces.tlb" no_namespace raw_interfaces_only named_guids

IDotNetMdiSupportPtr m_pForm;
IDotNetSupportBasePtr m_pFormBase;

HRESULT hr, hr2

hr =m_pForm.CreateInstance((LPCSTR)"<prefix>.BookAndPlanFormSupport");
hr2=m_pFormBase.CreateInstance((LPCSTR)"<prefix>.BookAndPlanFormSupport");

Here the m_pForm.CreateInstance succeeds, but m_pFormBase.CreateInstance fails with the error: "The system could not find the file specified".

To make things stranger, this is only a problem when I try to install the program, when I run it on my development machine everything works fine.

Has anybody experienced anything like this?
.

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