I cannot see the Nginx bad gateway error but I can see some errors in your logs:
/opt/bitnami/apps/bitnami/banner/htdocs//css/normalize.css" failed (2: No such file or directory)
Did you remove that files?
Also there seems to be errors with
Could you please describe what you did before the issues? Did you remove the files under
/opt/bitnami/apps/bitnami/banner, did you install any plugin?