Translation of plugins in mu-plugins directory


I've added the Signup TOS plugin in my mu-plugins folder. I generated a POT file and translated this into dutch.
I've added a text domain to the __ and _e calls in the code of signup-tos.php and generated a translated mo file.
I've also added this line in signup-tos.php :

load_plugin_textdomain('TOS', '/var/www/vhosts/');

But the plugin isn't translated in the frontend and in the backend.

Any ideas what I missed ?

kind regards,