XC21 Vref Error A

I have a system that uses an MD4 and some XC21 expansion modules. I am receiving an error code on the MD4 stating there is a VREF error A on XC21[0].

Currently our system uses the XC21 as digital inputs. We have a system power supply that supplies 24Vdc to a switch and then the digital input goes to the other side of the switch. This system has been running fine for multiple years, but now we have the Vref error. 

Since I am not actually using the Vref + at all, I know it isn't overloading. That seems to be the most common answer, as the XC21 Vrefs can only output 70 mA. Is the error bring prompted because something internally is damaged and needs to be replaced? 

Her is a picture of our wiring. C5 and C7 are the Vref A + signal, which aren't wired. C2, C4, C6, and C8 are all digital inputs from our system, all being 24Vdc. The system 24VDC is also powering the MD4 and XC21 so there isn't a concern about different power sources. 

Any thoughts?