Finally I have found a solution:

I added an alignment before some structures:

__declspec(align(8)) typedef struct
{
float iso_speed;
float shutter;
float aperture;
float focal_len;
time_t timestamp;
unsigned shot_order;
unsigned gpsdata[32];
char desc[512],
artist[64];
} libraw_imgother_t;

__declspec(align(8)) typedef struct
{
unsigned int progress_flags;
unsigned int process_warnings;
libraw_iparams_t idata;
libraw_image_sizes_t sizes;
libraw_colordata_t color;
libraw_imgother_t other;
libraw_thumbnail_t thumbnail;
libraw_rawdata_t rawdata;
ushort (*image)[4] ;
libraw_output_params_t params;
void *parent_class;
} libraw_data_t;

Everything works fine now and I am getting all correct metadata values (ISO...).

I guess, the default structure fields alignment on VS2010 is 8 bytes and not 4 bytes.

Thanks for your help :-)