Hi,
Can you check if the request is entering into SAP ECC? Maybe the error is just an exception in your RFC and this Exception is not waited in your Operation Mapping.
It´s just a suggestion,check ST22 in ECC, maybe you are lucky. Also put a session break-point at the begining of your RFC just for checking (remember use the same user that you communication channel is configured and it must be a dialog user).
I had a similar issue and the problem was a exception in RFC (SAP ECC) and RFC was not prepared to send exception.
The error was: Mapping program not found and we was looking in SAP PI cache etc...
Thanks.