==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release): iRedMail-1.7.4
- Deployed with iRedMail Easy or the downloadable installer? Downloadable
- Linux/BSD distribution name and version: Debian 12
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): PGSQL
- Web server (Apache or Nginx): Nginx
- Manage mail accounts with iRedAdmin-Pro? : What?
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.
====
I was trying to open RoundCube for accesing my first mail accoutn but the server sent me some errors like:

connect() failed (111: Connection refused) while connecting to upstream, client: ..., server: _, request: "GET /mail/ HTTP/2.0", upstream: "fastcgi://127.0.0.1:9999", host: "mail.insoli.cl"

So, with the help of Copilot, reached the file
/etc/nginx/templates/fastcgi_php.tmpl
Where replaced:

# Handle PHP files with upstream handler
#fastcgi_pass php_workers; <--- Replaced this, into the line below
fastcgi_pass unix:/run/php/php-fpm.sock;

And restarted NGINX

So, Roundcube launched with no trouble.

Created the topic for future reference (lost 2 hours/2 reinstalls resolving this)