Errors when calling DLL

I get the following errors when calling the following methods in a DLL

#ifdef MTPHELPER_EXPORTS
#define MTPHELPER_API __declspec(dllexport)
#else
#define MTPHELPER_API __declspec(dllimport)
#endif

extern "C"
{

// Initialize MTP helper library
MTPHELPER_API HRESULT MtpInitialize();

// Start playing
MTPHELPER_API HRESULT MtpPlay();

...<snip>

I get an
"org.jawin.COMExcpetion: 80004005: Unspecified error" - when I'm trying to
MtpInitialize
and
"COMExcpetion: 8000ffff: Catastrophic failure" - when I'm trying to MtpPlay

I use the following Java code

FuncPtr f = new FuncPtr("MtpHelper.dll", "MtpInitialize");
f.invoke(ReturnFlags.CHECK_HRESULT);

Any idea what I'm doing wrong??

Thanks
Thomas