For Serial cables, only pins 2, 3, and 5 are used on the RFX9600. Unfortunately, you cannot forgo any of these 3 connections.

I am interested in what types of equipment you have connected to the RFX9600 via the mono cables. Your only solution may be to replace these cables with emitters. I would start by powering down everything and replacing the mono cables 1 by one. If you have more than 1 mono cable connected, then remove all cables and replace with emitters until you are left with 1 mono cable only. If you still have issues, then you may find that you need to do trial and error on switching out various mono cables.

I do know first-hand that the ground side of the IR ports is not isolated per port so if using Mono cables, you can get some bleed through between ports. I experienced this with Escient equipment and Belgium confirmed my suspicions of a shared ground.