Aplying image styles to images return 404 for images
I don't know what caused the issue but at the end I solved It following this steps:

http://www.aldibier.com/blog/articulo/drupal-7-cuando-no-aparecen-las-imagenes-generadas-por-image-style

1- Adding to the Nginx(): 

location @rewrite {
  # Some modules enforce no slash (/) at the end of the URL
  # Else this rewrite block wouldn't be needed (GlobalRedirect)
  rewrite ^ /index.php;
}

location ~ ^/sites/.*/files/styles/ {
  try_files $uri @rewrite;
}


2- Adding to the settings.php:

$conf['image_allow_insecure_derivatives'] = TRUE;


3- Rebooting the server. 

This solution may bypass the error since, for what I know, nothing in the server configuration was changed when the problem started so this shouldn't be the root of the problem.