Errores instalando WordPress

Forbidden
You don’t have permission to access /wp-admin/index-extra.php on this server.

Forbidden
You don’t have permission to access /wp-admin/admin-ajax.php on this server.

Solución:

añadir esto al .htaccess de wp-admin

<IfModule mod_security.c>
SecFilterInheritance Off
</IfModule>