403 error when importing from old WordPress site

Hi,

Does anyone know why I am getting a 403 error when importing everything from an old WP site via the WordPress importer?

I get

"Access is forbidden to the requested page:

/sitename/wp-admin/admin.php?import=wordpress&step=1&_wpnonce=(...) (port 80)"

http://wordpress.org/plugins/wordpress-importer/