Uncaught Twig_Error_Loader after an upgrade

This is a bit of an edge-case, but I think it's wise to investigate.

I'm staging my upgrade 3.1.10->3.2 on my DEV machine (Win10 with PHP 7.0.3 and MySQL 5.7).

I'm using the clean approach: clean up root, deploy phpBB 3.2 files, restore from backup images, ext, ecc, migrate via CLI (php.exe bin/phpbbcli.php --safe-mode db:migrate).

My phpBB 3.1 had the italian language pack installed (italian was set has default). phpBB 3.2 has not, since the language pack is not available yet.

Now, when a user with the italian language posts to a topic, and someone is subscribed to said topic's notifications, the poster gets this error in his browser:

{{[08-Jan-2017 00:10:34 Europe/Rome] PHP Fatal error: Uncaught Twig_Error_Loader: The "" directory does not exist. in <MY_FORUM_ROOT_PATH_REMOVED>vendor\twig\twig\lib\Twig\Loader\Filesystem.php:94
Stack trace:
#0 <MY_FORUM_ROOT_PATH_REMOVED>phpbb\template\twig\loader.php(110): Twig_Loader_Filesystem->addPath(false, '_main_')
#1 <MY_FORUM_ROOT_PATH_REMOVED>vendor\twig\twig\lib\Twig\Loader\Filesystem.php(76): phpbb\template\twig\loader->addPath('./language/it/e...', '_main_')
#2 <MY_FORUM_ROOT_PATH_REMOVED>phpbb\template\twig\twig.php(236): Twig_Loader_Filesystem->setPaths(Array)
#3 <MY_FORUM_ROOT_PATH_REMOVED>includes\functions_messenger.php(697): phpbb\template\twig\twig->set_custom_style(Array, Array)
#4 <MY_FORUM_ROOT_PATH_REMOVED>includes\functions_messenger.php(281): messenger->set_template_paths(Array, Array)
#5 <MY_FORUM_ROOT_PATH_REMOVED>phpbb\notification\method\messenger_base.php(100): messenger->template('topic_notify', 'en', './language/en/e...', '')
#6 D:\W in <MY_FORUM_ROOT_PATH_REMOVED>vendor\twig\twig\lib\Twig\Loader\Filesystem.php on line 94}}