Included it, now I get an return value, but all values in that structure are 0. Tried the same with C# and there the returned structure got values. So I am still doing something wrong. When I output the recieved object it looks like:

FFaceApi$INVENTORYITEM$ByValue(allocated@0x2e22690 (4 bytes)) { short id@0=0 byte index@2=0 byte count@3=0 }

To me that looks quite okay despite that id should carrry the value 17567.

What else can I try to get a properly set structure as return value?

The cause of the reported access violation was not forced by calling GetInventoryItem. Before this i do a function call for:

FFACE_API void GetItemNameByID(short index, void *buffer, int *size, DWORD PID = 0 );

Strange about it is, that depending upon the mapping I do get or not get an access violation. The mapping causing it looks like this:

void GetItemNameByID(short index, PointerByReference buffer, IntByReference size, int PID);

That function returns properly the string I expect by just using buffer.getPointer().getString(0) but the next call (which likely is GetInventoryItem) crashes.

By changing the mapping to

void GetItemNameByID(short index, byte buffer[], IntByReference size, int PID);

I still get the same text but the crash does not occur anymore.