@dc42 Apologies about the delay.
I received the 1HCL warranty replacement and installed it as before, but the connection problem persists; the MB6HC still doesn't recognize the expansion board, the STATUS LED on the 1HCL doesn't flash (the V_FUSED LED does light up solidly and the voltage indicators flash rapidly)
I attempted to diagnose the issue by seeing if there were any obvious problems such as powering the Mainboard and 1HCL with separate power supplies, checking the continuity of the RJ11 cable and voltage (there is continuity and voltage), cutting open the the RJ11 cable to see if their wires were twisted (they weren't), reflashing the Mainboard's firmware.
Nothing has worked and now I'm stumped.