This happened after WP was installed without SSL and then after some days, SSL was added. It only happened when trying to access /wp-admin/ route.

Adding the following lines of code at the after “/* Add any custom values between this line and the “stop editing” line. */” line of wp-config.php:

if (strpos($_SERVER['HTTP_X_FORWARDED_PROTO'], 'https') !== false)
   $_SERVER['HTTPS']='on';

In the proxy settings for nginx add these directives:

...
  location / {
      proxy_set_header Host            $host;
      proxy_set_header X-Forwarded-For $remote_addr;
      proxy_set_header X-Forwarded-Proto $scheme;
      proxy_pass http://your-wordpress-docker-upstream$request_uri;
    }
...

• WP installed in docker container behind nginx, with Letsencrypt https

• Wordpress on nginx

• https://wordpress.stackexchange.com/questions/302965/too-many-redirects-only-when-trying-to-access-wp-admin-page
• https://wordpress.org/support/article/administration-over-ssl/#using-a-reverse-proxy