Unable to access Wordpress Dashboard with nginx reverse proxy

I am facing a bit of an issue when setting up WordPress with Nginx as reverse proxy to Apache backend. All the pages are loading, but i get an error when i try to login to wp-admin dashboard. The error is Sorry, you are not allowed to access this page.

I have checked my file permissions, database prefix,.htaccess and even usermeta admin privileges in the db, all seem to be perfect. The site was working perfectly fine before i setup nginx reverse proxy.

Here is my apache2 config:

   <VirtualHost *:8081>
       DocumentRoot "/mnt/NAS/wp_data/wordpress/"
       ServerName my_site_url
       ServerAlias www.my_site_url
    
    <Directory "/mnt/NAS/wp_data/wordpress/">
       Options MultiViews FollowSymlinks
       AllowOverride All
       Order allow,deny
       Allow from all
    </Directory>
    
   </VirtualHost>

Here is my apache ports.conf:

   #Listen 80
   Listen 8081

Here is my nginx config:

server {
        listen 80;
        listen [::]:80;

        server_name my_site_url;

        rewrite ^ https://$server_name$request_uri? permanent;

}

server {
        listen 443 ssl;
        listen [::]:443 ssl;

        server_name www.my_site_url my_site_url;

        location / {
                proxy_set_header Host $host;
                proxy_set_header X-Real-IP $remote_addr;
                proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
                proxy_set_header X-Forwarded-Proto $scheme;
                proxy_pass http://127.0.0.1:8081;
        }

        ssl_certificate /var/www/mycert/certificate.pem;
        ssl_certificate_key /var/www/mycert/private.key;
}

Last but not least my wp-config.php is as default and i haven't added anything else apart from

if ( isset( $_SERVER['HTTP_X_FORWARDED_PROTO'] ) ) {
if ( 'https' == $_SERVER['HTTP_X_FORWARDED_PROTO'] ) {
$_SERVER['HTTPS'] = 'on';
}
}
if ( isset( $_SERVER['HTTP_X_REAL_IP'] ) ) {
$_SERVER['REMOTE_ADDR'] = $_SERVER['HTTP_X_REAL_IP'];
}

I was able run Wordpress and its dashboard on apache with nginx as reverse proxy

the nginx configuration is

 location ^~ /blog/ {
    proxy_pass http://x.y.x.z/;
    proxy_set_header Host $http_host;
    proxy_set_header X-Forwarded-Host $http_host;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header X-Forwarded-Proto $scheme;
}

and in the wp-config.php (on apache server) the following contents are added

$_SERVER['REQUEST_URI'] = str_replace("/wp-admin/", "/blog/wp-admin/",  $_SERVER['REQUEST_URI']);


if ( isset( $_SERVER['HTTP_X_FORWARDED_PROTO'] ) ) {
if ( 'https' == $_SERVER['HTTP_X_FORWARDED_PROTO'] ) {
$_SERVER['HTTPS'] = 'on';
}
}
if ( isset( $_SERVER['HTTP_X_REAL_IP'] ) ) {
$_SERVER['REMOTE_ADDR'] = $_SERVER['HTTP_X_REAL_IP'];
}