If you can no longer log into Kibana after upgrading to Elasticsearch 5.6.0, you might be affected by a known issue. Clusters originally created in version 2.x and later upgraded to version 5.x before upgrading to 5.6.0 are known to be affected.
We recommend that you do NOT upgrade your cluster to version 5.6.0 if your cluster was originally created in version 2.x and later upgraded to 5.x. If you have already upgraded and are affected, email firstname.lastname@example.org
and we will get you back on track.
We are working on resolving the cause of the issue as quickly as possible.