Private files won't be saved anymore in their intended location? (D7)
You spoted your problem in your apache log file : Apache is forbiding access.  

You have 2 options here :

If you want apache to allow access to these files then add this to apache conf file : 

&lt;Directory /home/sitio/public_html/files/private/curso&gt;
  AllowOverride None
  Order Allow,Deny
  Allow From All 
&lt;/Directory&gt;


then restart apache and I guess you should be ok to access your file.

But if you want these files to be private then you want to restrict access rather than open it, so  

Alternatively,  

you let apache restrict access to these files, and you set drupal serve these files via alternative drupal-generated links. Check the documentation for setting up access to private files here, look for the section on "Accessing Private Files".

If you've got no .htaccess file for the folder as you said then create one with :

Deny from all
Options None
Options +FollowSymLinks