Nginx reverse proxy and wordpress

Everything working well but I'm getting an issue with WordPress while I'm in the extensions page.

The problem is that plugin icons are not getting displayed and when I click on the plugin icons of the plugin which I want to install, it opens up an empty window which keeps loading forever. But when I click on "install" button, the plugin installs without any issue.

You can look this image to see what happen

Issue Wordpress

Here the config :

Config

Here is the Webserver config:

Front end Nginx

server {
        listen 443 ssl;

            # SSL 
            ssl on;
            ssl_certificate /etc/ssl/nginx/nginx.crt;
            ssl_certificate_key /etc/ssl/nginx/nginx.key;

            ssl_session_cache shared:SSL:40m;
            ssl_session_timeout 10m;
            ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
            ssl_prefer_server_ciphers on;

        server_name domaine.tld;

        # Proxy Pass to Varnish and  Add headers to recognize SSL
        location / {
            proxy_pass  http://127.0.0.1:80;

            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
            proxy_set_header X-Forwarded-Host $host;
            proxy_set_header X-Real-IP $remote_addr;
            proxy_set_header X-Forwarded-Proto https;
            proxy_set_header X-Forwarded-Port 443;
            proxy_set_header X-Secure on;
        }
}

Backend Nginx

server {
        listen 8000;

        server_name domaine.tld;
        root /var/www/domaine;
        index index.php;

        # Custom Error Page

        error_page  404 403 /page_error/404.html;
        # Log
        access_log /var/log/nginx/access.log;
        error_log /var/log/nginx/error.log;

        location / {
            try_files $uri $uri/ /index.php?$args;
                    auth_basic "Restricted";
                auth_basic_user_file /etc/nginx/.htpasswd;
        }


            # PHP-FPM
            location ~ \.php$ {
                try_files $uri =404;
                    fastcgi_split_path_info ^(.+\.php)(/.+)$;
                    include fastcgi_params;
                    fastcgi_index index.php;
                    fastcgi_param SCRIPT_FILENAME         $document_root$fastcgi_script_name;
                    fastcgi_pass unix:/var/run/php5-fpm.sock;
                    fastcgi_param  HTTPS              on;

            }
}

Varnish Default

DAEMON_OPTS="-a :80 \
             -T localhost:6082 \
             -f /etc/varnish/default.vcl \
             -S /etc/varnish/secret \

             -s malloc,256m"

Varnish VCL 4.0

backend default {
    .host = "127.0.0.1";
    .port = "8000";
    .connect_timeout = 600s;
    .first_byte_timeout = 600s;

    .between_bytes_timeout = 600s;
    .max_connections = 800;
}

# Only allow purging from specific IPs
acl purge {
    "localhost";
    "127.0.0.1";
}


# This function is used when a request is send by a HTTP client (Browser) 
sub vcl_recv {

    # Redirect to https
    if ( (req.http.host ~ "^(?i)www.domaine.tld" || req.http.host ~ "^(?i)domaine.tld") && req.http.X-Forwarded-Proto !~ "(?i)https") {
                return (synth(750, ""));
        }

    # Normalize the header, remove the port (in case you're testing this on various TCP ports)
    set req.http.Host = regsub(req.http.Host, ":[0-9]+", "");


    # Allow purging from ACL
    if (req.method == "PURGE") {
        # If not allowed then a error 405 is returned
        if (!client.ip ~ purge) {
            return(synth(405, "This IP is not allowed to send PURGE requests."));
        }   
        # If allowed, do a cache_lookup -> vlc_hit() or vlc_miss()
        return (purge);
    }


    # Post requests will not be cached
    if (req.http.Authorization || req.method == "POST") {
        return (pass);
    }

    # Did not cache .ht* file
    if ( req.url ~ ".*htaccess.*" ) {
        return(pass);
    }


    if ( req.url ~ ".*htpasswd.*" ) {
        return(pass);
    }

    # Don't cache phpmyadmin
    if ( req.url ~ "/nothingtodo" ) {
        return(pass);
    }


    # --- Wordpress specific configuration

    # Did not cache the RSS feed
    if (req.url ~ "/feed") {
        return (pass);
    }

    # Don't cache 404 error 
    if (req.url ~ "^/404") {
        return (pass);

    }

    # Blitz hack
        if (req.url ~ "/mu-.*") {
                return (pass);
        }


    # Did not cache the admin and login pages
    if (req.url ~ "/wp-(login|admin)") {

        return (pass);
    }

     # Do not cache the WooCommerce pages
     ### REMOVE IT IF YOU DO NOT USE WOOCOMMERCE ###
    if (req.url ~ "/(cart|my-account|checkout|addons|/?add-to-cart=)") {
            return (pass);
        }

    # First remove the Google Analytics added parameters, useless for our backend

    if(req.url ~ "(\?|&)(utm_source|utm_medium|utm_campaign|gclid|cx|ie|cof|siteurl)=") {
        set req.url = regsuball(req.url, "&(utm_source|utm_medium|utm_campaign|gclid|cx|ie|cof|siteurl)=([A-z0-9_\-\.%25]+)", "");
        set req.url = regsuball(req.url, "\?(utm_source|utm_medium|utm_campaign|gclid|cx|ie|cof|siteurl)=([A-z0-9_\-\.%25]+)", "?");
        set req.url = regsub(req.url, "\?&", "?");
        set req.url = regsub(req.url, "\?$", "");
    }

    # Remove the "has_js" cookie
    set req.http.Cookie = regsuball(req.http.Cookie, "has_js=[^;]+(; )?", "");


    # Remove any Google Analytics based cookies
    set req.http.Cookie = regsuball(req.http.Cookie, "__utm.=[^;]+(; )?", "");

    # Remove the Quant Capital cookies (added by some plugin, all __qca)
    set req.http.Cookie = regsuball(req.http.Cookie, "__qc.=[^;]+(; )?", "");

    # Remove the wp-settings-1 cookie
    set req.http.Cookie = regsuball(req.http.Cookie, "wp-settings-1=[^;]+(; )?", "");

    # Remove the wp-settings-time-1 cookie

    set req.http.Cookie = regsuball(req.http.Cookie, "wp-settings-time-1=[^;]+(; )?", "");

    # Remove the wp test cookie
    set req.http.Cookie = regsuball(req.http.Cookie, "wordpress_test_cookie=[^;]+(; )?", "");

    # remove cookies for comments cookie to make caching better.
    set req.http.cookie = regsub(req.http.cookie, "dcd9527364a17bb2ae97db0ead3110ed=[^;]+(; )?", "");

    # remove ?ver=xxxxx strings from urls so css and js files are cached.
    set req.url = regsub(req.url, "\?ver=.*$", "");

    # Remove "replytocom" from requests to make caching better.
    set req.url = regsub(req.url, "\?replytocom=.*$", "");
    # Strip hash, server doesn't need it.
    set req.url = regsub(req.url, "\#.*$", "");
    # Strip trailing ?
    set req.url = regsub(req.url, "\?$", "");

    # Are there cookies left with only spaces or that are empty?
    if (req.http.cookie ~ "^ *$") {
            unset req.http.cookie;

    }

    # Drop any cookies sent to Wordpress.
    if (!(req.url ~ "wp-(login|admin)")) {
                       unset req.http.cookie;
        }

    # Cache the following files extensions 
    if (req.url ~ "\.(css|js|png|gif|jp(e)?g|swf|ico)") {
        unset req.http.cookie;

    }

    # Normalize Accept-Encoding header and compression
    # https://www.varnish-cache.org/docs/3.0/tutorial/vary.html
    if (req.http.Accept-Encoding) {
        # Do no compress compressed files...
        if (req.url ~ "\.(jpg|png|gif|gz|tgz|bz2|tbz|mp3|ogg)$") {
                unset req.http.Accept-Encoding;
        } elsif (req.http.Accept-Encoding ~ "gzip") {
                set req.http.Accept-Encoding = "gzip";

        } elsif (req.http.Accept-Encoding ~ "deflate") {
                set req.http.Accept-Encoding = "deflate";
        } else {
            unset req.http.Accept-Encoding;
        }
    }

    # Check the cookies for wordpress-specific items
    if (req.http.Cookie ~ "wordpress_" || req.http.Cookie ~ "comment_") {
        return (pass);

    }
    if (!req.http.cookie) {
        unset req.http.cookie;
    }

    # --- End of Wordpress specific configuration

    # No cache for big video files
    if (req.url ~ "\.(avi|mp4)") {
        return (pass);

    } 

    # Did not cache HTTP authentication and HTTP Cookie
    if (req.http.Authorization || req.http.Cookie) {
        # Not cacheable by default
        return (pass);
    }

    # Cache all others requests
    return (hash);

}

sub vcl_pipe {
    # Note that only the first request to the backend will have
    # X-Forwarded-For set. If you use X-Forwarded-For and want to
    # have it set for all requests, make sure to have:
    # set bereq.http.connection = "close";
    # here. It is not set by default as it might break some broken web
    # applications, like IIS with NTLM authentication.
    #set bereq.http.Connection = "Close";

    return (pipe);
}

sub vcl_pass {
    return (fetch);
}

sub vcl_synth {
    if (resp.status == 750) {
        set resp.status = 301;

        set resp.http.Location = "https://www.paris-vendome.com" + req.url;
        return(deliver);
    }
}


# The data on which the hashing will take place
sub vcl_hash {
    hash_data(req.url);
    if (req.http.host) {

        hash_data(req.http.host);
    } else {
        hash_data(server.ip);
    }

    # hash cookies for requests that have them
    if (req.http.Cookie) {
        hash_data(req.http.Cookie);
    }


    # If the client supports compression, keep that in a different cache
        if (req.http.Accept-Encoding) {
            hash_data(req.http.Accept-Encoding);
    }

    return (lookup);
}

# This function is used when a request is sent by our backend (Nginx server)
sub vcl_backend_response {

    # Remove some headers we never want to see
    unset beresp.http.Server;
    unset beresp.http.X-Powered-By;

    # For static content strip all backend cookies
    if (bereq.url ~ "\.(css|js|png|gif|jp(e?)g)|swf|ico") {
        unset beresp.http.cookie;
    }

    # Only allow cookies to be set if we're in admin area

    if (beresp.http.Set-Cookie && bereq.url !~ "^/wp-(login|admin)") {
            unset beresp.http.Set-Cookie;
        }

    # don't cache response to posted requests or those with basic auth
    if ( bereq.method == "POST" || bereq.http.Authorization ) {
            set beresp.uncacheable = true;
        set beresp.ttl = 120s;
        return (deliver);
        }


        # don't cache search results
    if ( bereq.url ~ "\?s=" ){
        set beresp.uncacheable = true;
                set beresp.ttl = 120s;
                return (deliver);
    }

    # only cache status ok
    if ( beresp.status != 200 ) {

        set beresp.uncacheable = true;
                set beresp.ttl = 120s;
                return (deliver);
    }

    # A TTL of 24h
    set beresp.ttl = 24h;
    # Define the default grace period to serve cached content
    set beresp.grace = 30s;


    return (deliver);
}

# The routine when we deliver the HTTP request to the user
# Last chance to modify headers that are sent to the client
sub vcl_deliver {
    if (obj.hits > 0) { 
        set resp.http.X-Cache = "cached";
    } else {
        set resp.http.x-Cache = "uncached";

    }

    # Remove some headers: PHP version
    unset resp.http.X-Powered-By;

    # Remove some headers: Apache version & OS
    unset resp.http.Server;

    # Remove some headers: Varnish
    unset resp.http.Via;

    unset resp.http.X-Varnish;

    unset resp.http.Age;
    unset resp.http.Link;

    return (deliver);
}

sub vcl_hit {
    return (deliver);

}
sub vcl_miss {
    return (fetch);
}

sub vcl_init {
    return (ok);
}

sub vcl_fini {

    return (ok);
}

I think that the issue is not related to varnish, but with the backend, because when I test with this config ( no varnish / no backend ), everything works without any issue:

server {
       listen         80;
       server_name    domaine.tld;
       return         301 https://www.domaine.tld$request_uri;

}


server{
    listen 443;
    ssl on;
    ssl_certificate /etc/ssl/nginx/nginx.crt;
    ssl_certificate_key /etc/ssl/nginx/nginx.key;
    ssl_session_timeout 10m;


    root /var/www/domaine;
    index index.htm index.html index.php;

    server_name domaine.tld;

    server_tokens off;
    access_log /var/log/nginx/access.log;
    error_log /var/log/nginx/error.log;

    error_page 404 403         /page_error/404.html;

    error_page 500 502 503 504 /page_error/50x.html;

    gzip on;
    etag off;


    location / {
        try_files $uri $uri/ =404;
        auth_basic "Restricted";
        auth_basic_user_file /etc/nginx/.htpasswd;

    }

    location ~ \.php$ {


        try_files $uri =404;
        fastcgi_split_path_info ^(.+\.php)(/.+)$;
        include fastcgi_params;
        fastcgi_index index.php;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;

        fastcgi_pass unix:/var/run/php5-fpm.sock;
        fastcgi_param  HTTPS              on;
    }
}

If I have missed anything or if you would like more information, please does not hesitate.
Sorry for the big post but I'm desperate.
Hope somebody will help me
Thanks in advance