Kibana 5.6.2 is released today with a critical fix to the Upgrade Assistant in X-Pack basic as well as a fix for a metric visualization regression.
The change to the Upgrade Assistant is critical for any users that started using Kibana prior to 5.5 and intend to upgrade to 6.0. The Upgrade Assistant helps users migrate their .kibana index to a single-type format that is compatible with Elasticsearch 6.0, but prior to version 5.6.2 it would not properly disable dynamic mappings for the .kibana index after migration, so saving an object in Kibana would result in an unexpected new type in the .kibana index, which would put Kibana into a permanent red status.
If your Kibana install began in 5.5 or later, or if you haven't run the upgrade assistant yet, just go ahead and upgrade to 5.6.2 to make sure this issue is fixed going forward.
If your Kibana install predates 5.5, and you've already upgraded the .kibana index via the Upgrade Assistant in 5.6.0 or 5.6.1, then you must follow a manual process to fix the data corruption issue, which is documented in this forum post, and you should upgrade to 5.6.2 as well.
The regression with metric visualizations did not respect style properties that were set on existing metric visualizations. Upgrading to 5.6.2 should resolve this issue.