Problem with permalink

Hello,

I created a network for a multilingual website. (en + fr)
http://www.exemple.com
http://www.exemple.com/fr

I have a menu with different links:
http://www.exemple.com/about
http://www.exemple.com/projects
http://www.exemple.com/contact
…

In the “Permlinks” option if I check “Post name”, I cannot navigate…
Every time I am redirected to the home page.
If I check the default option (?p=123), there is no problem.

This problem only affects my main website (www.exemple.com), on the french version, “Post name” works perfectly.

I suspect a problem with the .htaccess, but I can not find the error.

Here is my code:
.htaccess

RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [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]

wp-config.php

define('WP_DEBUG', false);

/* Multisite */
define( 'WP_ALLOW_MULTISITE', true );

define('MULTISITE', true);
define('SUBDOMAIN_INSTALL', false);
define('DOMAIN_CURRENT_SITE', 'example.fr');
define('PATH_CURRENT_SITE', '/');
define('SITE_ID_CURRENT_SITE', 1);
define('BLOG_ID_CURRENT_SITE', 1);

Thanks in advance.
Regards.
Nicolas