We have 4 systems all with this same problem. IPMI reporting CPUs overheating, fans rev up and down. Viewer does not report any temp for the CPU, just N/A on all 4 units.
Please download IPMICFG utility from https://www.supermicro.com/wdl/utility/IPMICFG/ to your local hard drive and use "ipmicfg -fd" command to reset BMC to factory default.