The web form for creating a new custom username alias won't allow a custom user name of the form:
'FirstName LastName firstname.lastname@example.org' - without the quotes.
To my understanding a validation error occurs, having no description.
Well, false alarm. It works for me too. I just thought that those icons (red circle with a minus sign) were indicating validation errors, as they look like the 'do not enter' traffic sign.
Sorry for spending your time without a reason.