Can't upload images

I migrate my blog from other hosting with the multisite and now I can’t upload any image.

It always appears an error like “It was impossible to create the directory uploads/2014/06. Make sure that the server have the right write permissions for the upper directory” (it’s translated from the error thata appears to me in Spanish -> “No se pudo crear el directorio uploads/2014/06. Asegúrate de que el servidor tiene permisos de escritura para el directorio superior.”).

I suspect that the problem could be in the htaccess, this is what I have in it:

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# uploaded files
RewriteRule ^([_0-9a-zA-Z-]+/)?files/(.+) wp-includes/ms-files.php?file=$2 [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]

</IfModule>

But I don’t have too much idea of what do I have to change or why. I tried changing the permissions of the folders from 755 to 777 but the error still there.

Thanks in advance.