Hi LuigiC
Sorry for the basic questions, it's just to discard options,
1. Are you sure you are connecting your analyzer in the respective pins?
2. Do you have the GND of all channels connected?
3. Have you tried changing the mraa-mode function? Mraa-SPI functions
4. Under Decoded Protocols (In your logic analyzer), Are you able to see if there are any change in MISO or MOSI in time? In the picture, it seems that MOSI channel maintains it's value at HIGH and MOSI as low, try sending a data package with the function write(data*, lenght) to see if in some moment this change.
Let me know the outcome of this.
Regards;
CMata