It’s a little bit strange normally the C series should work with the UCS special agent.
Some of my installations also have C series devices and i get valid responses.
I had a look only difference i have here → all the C220 are M4 models.
I found also two Business Edition 6000h M5 rack systems and they are working without problem with SNMP:
You can try the Redfish test agent, it should give some data back 
The error message itself only indicates that the XML structure don’t know any class named “equipmentFanModuleStats”.
I think the special agent don’t recognize the system as a C class. The query for “equipmentFanModuleStats” should only be made if you have a B class.
Your model “APIC-SERVER-L3” is not matching the any model string in the special agent.
Strange is that he tries to fetch data then 
One solution would be to extend the regex for the C class to match this model then some output should be visible.