Graphics Interop unspecified driver error

Hello,
under Windows 7 64bit with VS2008 & CUDA 4.0 I am trying to generate two buffers for data position and color (just reporting the main):

int main(int argc, char** argv) {
cudaGLSetGLDevice(0);
init_data();
printf(“Device data inited\n”);

uint size=ITEMS*sizeof(float4);

glutInit( &argc, argv);
glutInitDisplayMode( GLUT_RGBA | GLUT_DOUBLE);
glutInitWindowSize( window_width, window_height);
glutCreateWindow( "Spiking Neural Network");
printf("GL context inited\n");
if( CUTFalse == initGL()) return 99;

glutDisplayFunc(display);
glutKeyboardFunc(keyboard);
glutMouseFunc(mouse);
glutMotionFunc(motion);

// create BOs
glGenBuffers( 1, &vbo);
glBindBuffer( GL_ARRAY_BUFFER, vbo);
glBufferData( GL_ARRAY_BUFFER, size, 0, GL_DYNAMIC_DRAW);
glBindBuffer( GL_ARRAY_BUFFER, 0);
printf("%s\n",cudaGetErrorString(cudaGraphicsGLRegisterBuffer(&vbo_cuda, vbo, cudaGraphicsMapFlagsWriteDiscard )));

glGenBuffers( 1, &cbo);
glBindBuffer( GL_ARRAY_BUFFER, cbo);
glBufferData( GL_ARRAY_BUFFER, size, 0, GL_DYNAMIC_DRAW);
glBindBuffer( GL_ARRAY_BUFFER, 0);

printf("%s\n",cudaGetErrorString(cudaGraphicsGLRegisterBuffer(&cbo_cuda, cbo, cudaGraphicsMapFlagsWriteDiscard )));

glutMainLoop();

}

The result is this:
C:\ProgramData\NVIDIA Corporation\NVIDIA GPU Computing SDK 4.0\C\bin\win32\Release>neutral.exe
Device data inited
GL context inited
unspecified driver error
unspecified driver error
BO Created

the “unspecified driver error” items are related to the cudaGraphicsGLRegisterBuffer.
I cannot understand what is the mistake I am doing…
Any hint / help is very appreciated!
Thankyou