Authentik IDP forward auth issue

[hooray4me]

I use Authentik as my identity provider for all my apps. I have Heimdall frontend'ed by Nginx Proxy Manager. This works fine. When I try to put in the required Advanced code in NPM to use Authentik, I get a 500 error. I suspect it's because nginx is also inside the container.

Can someone tell me what I would need to change in the container config to allow this to work? The nginx error log shows: auth request unexpected status: 400 while sending to client,

proxy_buffer_size 32k;

location / {
    proxy_set_header X-Forwarded-Proto https;
    proxy_pass          $forward_scheme://$server:$port;

    auth_request        /outpost.goauthentik.io/auth/nginx;
    error_page          401 = @goauthentik_proxy_signin;
    auth_request_set $auth_cookie $upstream_http_set_cookie;
    add_header Set-Cookie $auth_cookie;

    auth_request_set $authentik_username $upstream_http_x_authentik_username;
    auth_request_set $authentik_groups $upstream_http_x_authentik_groups;
    auth_request_set $authentik_email $upstream_http_x_authentik_email;
    auth_request_set $authentik_name $upstream_http_x_authentik_name;
    auth_request_set $authentik_uid $upstream_http_x_authentik_uid;

    proxy_set_header X-authentik-username $authentik_username;
    proxy_set_header X-authentik-groups $authentik_groups;
    proxy_set_header X-authentik-email $authentik_email;
    proxy_set_header X-authentik-name $authentik_name;
    proxy_set_header X-authentik-uid $authentik_uid;

}

location /outpost.goauthentik.io {
    proxy_pass          http://auth.domain.com/outpost.goauthentik.io;
    proxy_set_header    Host $host;
    proxy_set_header    X-Original-URL $scheme://$http_host$request_uri;
    add_header          Set-Cookie $auth_cookie;
    auth_request_set    $auth_cookie $upstream_http_set_cookie;

    proxy_pass_request_body off;
    proxy_set_header Content-Length "";
}

location @goauthentik_proxy_signin {
    internal;
    add_header Set-Cookie $auth_cookie;
    return 302 /outpost.goauthentik.io/start?rd=$request_uri;
}```