Ther username is no longer suppurted and has been changed to e-mail

You can find all changes from v14 in the conversion manual with reasons why it's changed.

https://helpcenter.websitex5.com/en/post/154023

Access management
In the Professional edition of version 14, the user registration process has
been optimized and it is now possible to manage online store customer
registration.
WHAT CHANGES
● To make sure users are unequivocally registered, their e-mail address
is used, instead of a username.
● User cards now have both a Name and Surname field, instead of just a
single Name field.
● It is no longer necessary to specify the page the user lands on after
logging in.
ADVANTAGES
● You can now manage manually-registered users in the same way as
automatically-registered users.
● You will now find customers who have made a purchase from your
online store in the group of automatically-registered users.

Best Regards,

Andre

Dear Richard, 

as Andre mentioned, the users have now to log in to the locked pages using their e-mails instead of the username, so to avoid problem which could have been caused by two similar usernames and to make therefore the process more secure.

Please let me know if I can be of any help.