Do you have any idea why or what the exact problem is?
Yes, I have. According to the manual the default baud rate is 115200 baud. You have to set it to 9600 Baud explicitly using the supplied tool. If you want to continue to use the SoftwareSerial to talk to the board you may want to decrease that value to 4800 as that's the maximum value SoftwareSerial is reliable in my experience. Keep in mind that you have to toggle the UART selection switch to be able to use the supplied software tool.