When upgrading ES and Kibana from 5.6 to 6.7, Kibana does not start up, because the migration of configuration index (.kibana) fails. The reason is rest.action.multi.allow_explicit_index = false setting.
It shouldn't be necessary to pass the index in the url since the index is also passed in the body of the _bulk request.
From the Bulk documentation
The endpoints are /_bulk , /{index}/_bulk , and {index}/{type}/_bulk . When the index or the index/type are provided, they will be used by default on bulk items that don’t provide them explicitly.
I'm not quite sure why it failed for you though but I'm glad you got it working!
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