Hello, and welcome to the 5.6.1 release of Kibana!
The 5.6.1 release of the Elastic Stack has an important fix for an issue affecting users upgrading to 5.6.0 with indexes created in Elasticsearch 2.x and earlier where Kibana becomes unavailable.
Who is affected
Anyone upgrading a cluster that has indexes created with Elasticsearch 2.x or prior. This does not affect users installing a fresh 5.6 cluster nor those who started their Elastic journey with the 5.x releases.
The problem is most visible through Kibana. After attempting to login to Kibana post-upgrade, the following error appears:
It may also manifest as the following error:
Or if X-Pack is in use:
In any case, the Elasticsearch cluster itself (including all of your data in Elasticsearch) is not affected and correctly reports itself as in Green status.
Upgrading to 5.6.1 is the best solution to the issue. You can read the details of the problem and about mitigation if you are already experiencing the issue in this forum post. If you are an Elastic Cloud user then Elastic Cloud support is working to mitigate this issue for you.