Nginx rewrite not working at all - mod-rewrite

Question

I am trying to set up a Piwigo to run on Nginx but I got into some issues with links that should contain and index.php like this:
foto.domain.net/index.php/category/3
It seems like a simple task, rewriting /index/category/3 to /index.php/category/3, but no matter what I write in those rewrites, I end up with a 404 error and a never changing error log entry that looks like this:
open() "/srv/http/foto/index/category/3" failed (2: No such file or directory), client: 94.242.246.23, server: ~^(?<prefix>www)?\.?(?<subdomain>[a-z]+)?\.domain\.net$, request: "GET /index/category/3 HTTP/1.1", host: "foto.domain.net", referrer: "http://foto.domain.net/"
The error log looks the same even if I add junk to the rewrite line such as:
rewrite ^/index((/|$).*)$ /JUNK_index.php$1 last;
So the rewrite part is faulty, somehow but I can't figure it out...
server {
listen 80;
server_name domain.net;
return 301 $scheme://www.domain.net$request_uri;
}
server {
listen 80;
server_name "~^(?<prefix>www)?\.?(?<subdomain>[a-z]+)?\.domain\.net$";
root /srv/http/$subdomain;
location #rewrites {
rewrite ^/picture((/|$).*)$ /picture.php$1 last;
rewrite ^/index((/|$).*)$ /index.php$1 last;
# The following is needed for batch operations which use i.php
rewrite ^/i((/|$).*)$ /i.php$1 last;
}
location /$subdomain {
index index.php index.html;
try_files $uri $uri.php $uri/ #rewrites;
}
location ~ ^(?<script_name>.+?\.php)(?<path_info>/.*)?$ {
try_files $script_name = 404;
fastcgi_pass unix:/var/run/php-fpm/php-fpm.sock;
fastcgi_param PATH_INFO $path_info;
fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
include fastcgi_params;
}
}

Answer 1

In general terms, as a checklist:
make sure the target/landing URI exists
this should have output "rewrite": nginx 2>&1 | grep -i -o rewrite
check your error and access log