SNMPv2-MIB:authenticationFailure - Loads of these from two of our switches?

Hello

As above really, they are coming from two new switches but the config seems to be the same as all the other switches we have in regards to the community strings.

Exact error:

cExtSnmpTargetAuthInetAddr = 10.51.20.5​1 
cExtSnmpTargetAuthInetType = 1 
authAddr = 10.51.20.5​1 
snmpTrapOID = SNMPv2-MIB​:authentic​ationFailu​re 
sysUpTime = 21 days 21 hours 54 minutes 45.15 seconds

Any ideas?

Thanks