Finally, I founded the solution :
If you see in the Apache log that kind of message :
PHP Warning: main() [<a href='function.main'>function.main</a>]: open_basedir restriction in effect. File(/usr/share/pear/fckeditor_php5.php) is not within the allowed path(s):
The following line added to .htaccess file in the root directory normaly solve the problem :
php_value include_path '.:/var/www/vhosts/domain.com/httpdocs'
The value is the root path of the domain for the web site. You can find it with phpinfo :
<?php phpinfo() ?>