If you disable Kerberos, change the Realm and enable Kerberos afterwards, then Ambari 2.0 versions still wants to use the old Realm.
This is a known bug => https://issues.apache.org/jira/browse/AMBARI-10930
But what to do if you cannot upgrade Ambari but needs to enable Kerberos again with a different Realm ?
You have to remove the remaining Kerberos artifacts by using the following REST call, then you can enable Kerberos with the new Realm applied:
curl -H "X-Requested-By:ambari" -u
<adminuser>:<adminpw> -X DELETE http://<ambari-server>:8080/api/v1/clusters/<clustername>/artifacts/kerberos_descriptor
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