.NET Programmatically accessing Exchange shared and public folders

I am trying to programmatically access Exchange shared and public folders via
Outlook from a .NET application.

Here is the line of code that is generating an error:

Dim olns As Outlook.NameSpace = OutlookInstance.GetNamespace("MAPI")
mapiFolder = olns.GetFolderFromID(folderID, storeID)

The error is:
"The messaging interface has returned an unknown error. If the problem
persists, restart Outlook".

I don't get the error message when accessing the user's default mailbox. I
also don't get the error message if the user manually opens Outlook before
using my application.

The application is correctly loading Outlook using:
' Create the instance of outlook
_OL = New Outlook.Application
I know this because I can see Outlook being added to the Task Manager.

I have been struggling with this for days so if anyone has ideas I would
appreciate them.

THANKS!