illegal character in username
As a temporary solution I'll change the error message to indicate that the name must be in English.
Thank you yossik, we'll check if WordPress allow this kind of usernames and then add it in our to-do list.
Thank you Martin,
I've checked it again using WordPress native "add user" and I got the same results.
So, It may be something in my setup and not a flow in wpForo.
WordPress was installed in English and then I've changed to local language.
There is no way to do that if WordPress doesn't allow.