Aplying image styles to images return 404 for images When image styles are aplied to images the modified images are not created and the image url returns a 404 error. 
I've been checking for solutions and none worked for me. I commented the Options +FollowSymLinks in the .htaccess
(https://www.drupal.org/node/1170440), I changed the files folder permissions to 777, I changed the theme and back... (image style (imagecache) images are not being created), and still nothing.

The worst part is that it was working fine until yesterday (at least i found it yesterday) and many of the pictures have their modified versions working, but not the new ones for which the url returns de 404.

Is there any thing I can do that I've not tried yet?

Thank you
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.