MAPI_E_FAILONEPROVIDER == 0x8004011D when accessing public folders using mfcmapi

I seem to be getting this error when selecting the public folder in mfcmapi. I'm using the Feb 2014 release of mfcmapi.

Code: MAPI_E_FAILONEPROVIDER == 0x8004011D
Function CallOpenMsgStore( lpMAPISession, (ULONG_PTR)m_hWnd, lpEntryID, ulFlags, (LPMDB*)lppMAPIProp)
File MainDlg.cpp
Line 437

I can access the private mailbox without issue using mfcmapi, but the public folders I'm not able to. This problem is causing my backup software not to do document level backups. So I need to resolve this issue.

Also, if I create another user and give them the same access as administrator, the new user cannot access both the private mailbox nor the public mailbox when using mfcmapi so it must be a rights issue, but I can't find where it is!

Any idea's would be appreciated.

Thanks