Ref. pg. 12 "To configure default Kerberos authentication"[1], the KrbHostFQDN should be the fully qualified domain name of the Impala server host you are trying to connect to. In this case, it should be "mycluster.lan".

I'd try that first and see if it work.

[1] https://docs.cloudera.com/documentation/other/connectors/impala-jdbc/2-6-12/Cloudera-JDBC-Driver-for...