Saw this in a user report.
The cluster in question lost its master and the state was rebuilt from that of the tservers (see KUDU-2902). In doing so, the master lost its IPKI cert info and a new record was generated.
After the Kudu cluster was operational, the existing Impala cluster could not issue queries. All queries failed with an error like this:
Restarting Impala fixed it.
I'm not sure if this is an issue with how Impala caches KuduClient instances, or if it's an issue with how the client itself caches the master's CA certificate. For now I'm assuming this is a Kudu issue and the client needs to detect this error and invalidate existing certificates.