Login to Admin not working for icecast server

If we compare your log entry with the default Nginx log format:

10.0.0.42 - [27/Aug/2018:18:16:05 +0000] "GET /admin/ HTTP/1.1" 401$remote_addr $remote_user [$time_local] "$request" $status

The - between the IP address and the time indicates the $remote_user variable was not sent with this request.

This variable takes the value of the user name supplied with the Basic authentication, so if looks like this is not being passed to your Icecast server.

What does your Nginx conf file look like? If you are routing the request to your icecast server with a proxy_pass directive you may need to specifically pass the auth header, like this:

proxy_set_header Authorization $http_authorization;

You may also find some of the paths within Icecast don't work properly, but if your Nginx build has the sub_filter module you can rewrite them, something like this:

sub_filter_types text/xhtml text/xml text/css;sub_filter 'href="/'  'href="<external-url>';sub_filter 'url(/'  'url(<external-url>';sub_filter_once off;sub_filter_last_modified on;proxy_set_header Accept-Encoding "";

There is no reason you shouldn't be able to access your admin pages through Nginx, it just takes a little tweaking. I run a production server in this way.