i had a similar problem with a C9500. Was a bug in the IOS-XE. The SNMP agent crashed when accessing certain OID. This should be visible in the (debug) log. I then blocked the corresponding OIDs in the SNMP view like this:
snmp-server view ALL iso included
snmp-server view ALL ciscoEntitySensorMIB excluded
snmp-server view ALL entSensorType excluded
snmp-server view ALL ciscoSlaInstanceId excluded
snmp-server view ALL ciscoSlaVersion excluded
snmp-server view ALL ciscoMgmt.771.1.1.2 excluded
snmp-server view ALL clauNotifEnable excluded
snmp-server view ALL ciscoSlaEnabled excluded
snmp-server view ALL ciscoTrustSecServerMIBNotifsControl excluded
snmp-server view ALL cepThroughputNotifEnabled excluded
snmp-server view ALL ciscoFlashFileEntry excluded
snmp-server group XXXXX v3 priv read ALL