Mate I don't know if this is the problem, but are you ale to open up both ends of your cable and jot down what wires go where. I know from my CNC engineering days that signals sent from the machine to the pc did not necessarily mean you had a good link. Sounds like you are getting info from the rfm but maybe not the other way.  On the CNC machines it was always easy to download data from the machine but uploading needed the correct null modem wiring and comm settings

I don't know what the errors codes represent so if you could find that out would obviously help. Also ensure that your comm settings are the same ie. the pc may be 9600,E,7,1 and the rfm is 9600,N,8,1 or something