Проблемы с CORS на Centos 7 с Nginx 1.12.2 на борту. Что может быть не так?

server {
        listen 80;
        server_name ;
        return 301 https://$server_name$request_uri;
}
server {
        charset utf-8;
        client_max_body_size 128M;

        autoindex on;
        server_tokens off;

        root   /../../frontend/current;
        access_log  /var/log/nginx/access.log  main;
        error_log  /var/log/nginx/error.log warn;

        listen 443 ssl;
        ssl_certificate /etc/letsencrypt/live/fullchain.pem;
        ssl_certificate_key /etc/letsencrypt/live/privkey.pem;
        include /etc/letsencrypt/options-ssl-nginx.conf;
        ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem;
        ssl_session_cache shared:SSL:50m;
        ssl_session_tickets off;
        ssl_stapling on;
        ssl_stapling_verify on;
        location / {
                try_files $uri $uri/ /index.html?$args;
                proxy_connect_timeout       300;
                proxy_send_timeout          600;
                proxy_read_timeout          600;
                proxy_buffer_size           64k;
                proxy_buffers               16 32k;
                proxy_busy_buffers_size     64k;
                proxy_temp_file_write_size  64k;
                proxy_pass_header           Set-Cookie;
                proxy_redirect              off;
                proxy_hide_header           Vary;
                proxy_set_header            Accept-Encoding '';
                proxy_ignore_headers        Cache-Control Expires;
                proxy_set_header            Referer $http_referer;
                proxy_set_header            Host   $host;
                proxy_set_header            Cookie $http_cookie;
                proxy_set_header            X-Real-IP  $remote_addr;
                proxy_set_header            X-Forwarded-Host $host;
                proxy_set_header            X-Forwarded-Server $host;
                proxy_set_header            X-Forwarded-For $proxy_add_x_forwarded_for;
        }
        location /api {
                add_header Strict-Transport-Security "max-age=15768000";
                add_header Access-Control-Allow-Origin $allow_origin;
                add_header Access-Control-Allow-Credentials 'true';
                add_header Access-Control-Max-Age $cors_max_age;
                add_header Access-Control-Allow-Methods $cors_allow_methods;
                add_header Access-Control-Allow-Headers $cors_allow_headers;
                set $initial_content_length $sent_http_content_length;
                add_header 'Content-Length' "";
                add_header 'Content-Length' $cors_content_length;
                set $initial_content_type $sent_http_content_type;
                add_header Content-Type "";
                add_header Content-Type $cors_content_type;
                if ($request_method = 'OPTIONS') {
                        return 204;
                }
                proxy_pass_request_headers      on;
                proxy_set_header        X-Real-IP $remote_addr;
                proxy_set_header        X-Forwarded-For $proxy_add_x_forwarded_for;
                proxy_set_header        Host $http_host;
                proxy_pass              http://127.0.0.1:3000/api/;
        }
        gzip on;
        gzip_disable "msie6";
        gzip_types text/plain text/css application/json application/x-javascript text/xml application/xml application/xml+rss text/javascript application/javascript;
        gzip_comp_level 5;

        location ~* ^.+\.(woff|rss|atom|jpg|jpeg|gif|png|ico|rtf)$ {
                expires max;
        }

        location ~ /.well-known {
                allow all;
        }
}
map $http_origin $allow_origin {
        default "$http_origin";
}
map $request_method $cors_method {
        default "allowed";
        "OPTIONS" "preflight";
}
map $cors_method $cors_max_age {
        default "";
        "preflight" 1728000;
}
map $cors_method $cors_allow_methods {
        default "";
        "preflight" "GET, POST, OPTIONS";
}
map $cors_method $cors_allow_headers {
        default "";
        "preflight" "Authorization,Content-Type,Accept,Origin,User-Agent,DNT,Cache-Control,X-Mx-ReqToken,Keep-Alive,X-Requested-With,If-Modified-Since";
}
map $cors_method $cors_content_length {
        default $initial_content_length;
        "preflight" 0;
}
map $cors_method $cors_content_type {
        default $initial_content_type;
        "preflight" "text/plain charset=UTF-8";
}