After lots of tests I've been able to isolate the error and I'm able to reproduce it. It has to do with localization. I've just got it on a new DNN install, following this steps:
1. Install DNN from scratch, choosing empty web as template and spanish (haven't tested other languages)
2. In Admin/Languages, add Catalan and Basque (they're two other languages spoken in certain regions of Spain). Leave Fallback Language as System Default
3. Nothing seems unusual ;-)
4. Go back to Admin/Languages and set Fallback Language, either for Catalan or for Basque, to Spanish.
5. Now, try to add a new user. Never mind if you use the Users>Add New User an Control Panel or the Add New User button in Users Admin or Superusers Admin: you won't be able to add the user and will get the same error as me.
Should I report this to Jira or I better wait for further checking?
Regards,
Francisco