Please don't imply a connection where there is none. The original question mentioned Elasticsearch 6.1 , and you are running 6.5.3. Not all the questions in the last message of @Christian_Dahlqvist are relevant now as X-Pack is already included but :
There is no specific default user created during installation in Elasticsearch. There are a number of built-in users but you first need to set their passwords in order to use them. Which user are you talking about ?
If you say "stops" working, then I am assuming it was working before? What was an indication that it worked before? Did you login with that user to Kibana via the login form?
Can you share some information on how you installed Elasticsearch (zip/tar archives ? .deb / .rpm ? docker? ) and what is your Operating System ?
This document instructs you to use nginx in front of Kibana to authenticate the users. This might have been a valid option when you didn't have security enabled but there really is no reason for you to keep nginx. Kibana can handle the authentication itself.
As you can see from the command you entered, this is not a Kibana user, it is an nginx user.
My suggestion is to remove nginx and follow our documentation for configuring security for the Elastic stack
Also, can you please modify the title to something more helpful for other people viewing / answering in these forums? Your problem has very little to do with role management and the issue you linked to in the first post.
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