This MS article may explain the cause of your grief :
Microsoft Knowledge Base Article - 290064

OL2002: GetSharedDefaultFolder Method Fails with Some Folder Types

It states:
You can use the GetSharedDefaultFolder method with the following folders:
Calendar
Tasks
Inbox
Contacts
Notes
Journal

These are the folders that are listed in the Outlook user interface when you assign delegate permissions to the folders.

In reply to Re: MS Outlook GetSharedDefaultFolder by NetWallah
in thread MS Outlook GetSharedDefaultFolder by Mitch

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