HI,
Are you seeing anything in your nginx access or error logs? What does the Kibana log file say? Make sure to set logging.verbose: true in your kibana.yml as well, so we can get detailed output. When I navigate to http://localhost:8080 , I am asked for a username/password. Once I enter it, nginx correctly forwards me to Kibana, which works as expected.
Here are few more such discuss issues with ngnix auth issues- hope it helps.
Thanks
Rashmi