Hi Rich,
can you run the same program once against the old N4S system and once against the new one? This will show us, where we need to look at for trouble-shooting:
a) If the program does not work in both cases, the problem is probably on external (C/C++ program) side. So we can take a closer look there, capture an RFC trace (and perhaps compare with the old librfc32 based program that still worked ok).
b) If it works in one case and does not work in the other case, then we need to look at the backend. Either something has changed in the new SAP release, or the new system is customized differently?! But in this case we (ABAP Connectivity) will probably not be able to help you: our field of expertise is the RFC communication layer, but I have no idea, how these XOM function modules work and what needs to be done on backend side in order to make them work. (You will probably have to open a support ticket for BC-XOM in that case.)
Best Regards, Ulrich