Special characters in user id

Hi, up until this morning, I’v been able to user WPUM with success, adding more than 900 users to my website. Among them, a large majority have ids that equals their email address. This simplifies the process of distinguishing external users from users from my company, for which a simple xyyyyy id format is used.

This morning, I had to restore yesterday’s database due to an attack on my website. The restore process was successful, but since then, I cannot add users with special characters in their id. I get an error message that says (in french) :

Erreur?:?cet identifiant n’est pas valide, car il utilise des caractères non autorisés. Veuillez utiliser un identifiant valide.

I’ve looked at all the WPUM parameters, I cannot find where one can prevent users to have special chars in their id. So where should I look ?