Nginx + Slim route POST data - php

Question

I have an issue configuring Nginx correctly to work with Slim POST route.
In a post here on StackOverflow it was said that it is not a good practice to rewrite POST request.
Post: nginx rewrite post data
The idea is to have http://someurl.com/scan which is a POST route in index.php and allow only POST requests on the /scan and deny everything else.
index.php:
$app->post('/scan', function(ServerRequestInterface $request, ResponseInterface $response) { ... }
My Nginx conf looks like this:
server {
listen 80;
server_name someurl.com;
index index.php;
root /var/www/maxime/public;
error_log /var/log/nginx/max.err
access_log /var/log/nginx/max.log
location / {
deny all;
}
location = /scan {
limit_except POST {
deny all;
}
rewrite ^/scan$ /index.php last;
}
location ~ index\.php$ {
try_files $uri =404;
fastcgi_split_path_info ^(.+\.php)(/.+)$;
fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
fastcgi_param SCRIPT_NAME $fastcgi_script_name;
include fastcgi_params;
fastcgi_pass php_pool;
fastcgi_keep_conn on;
}
}
To me this conf tells me that if a request is done on /scan and it is a POST request then we "redirect" everything to /index.php. Then the index.php location block comes into action which passes everything to the index.php or am I mistaking somewhere?
Currently with this setup I get this error in Nginx:
*100778 readv() failed (104: Connection re
set by peer) while reading upstream client: 123.123.123.123, server: someurl.com,
request: "POST /scan HTTP/1.1", upstream: "fastcgi://127.0.0.1:9001", host: "someurl.com"