I don't remember right off, but somehow you can get it to show the byte values that are sent and received. If you can't get that to work, we can go back to stepping through the code.
Open ModbusBase.vb and put a breakpoint at line 664, then run the application, When it stop at the break point, step through with F10 or F11 to see if it goes to line 655:
If PDU.ExceptionCode = 0 Then