Port 12 works yeah, and I've taken the MR33 that was on that and tried it on port 11 with same result. Now, I did ask the customer over the phone to take an MR33 and plug direct to port 11 thus bypassing in-wall cabling. They did, but same result. I'm keen to go onsite myself and do all this again from the top. Before that, I'll reboot the MX though.
Firmware is latest on all devices yeah.
I'm also suspicious about power. Everything in the whole utility closet is plugged into a $5 power strip from Moe's Power Strip Emporium or some such and I wonder if maybe it can't supply enough power anymore. So I'll play with toggling power, like having port 12 empty while troubleshooting with port 11.
Lastly yeah it could be the actual port perhaps - I'll reserve that conclusion for when all else fails I suppose or I can try to see if I have a wire mapper/tester handy.
I shall check back in a few days.