If you clone a user, for example, peter in matteo, you will keep under matteo permissions (acl_users) write permissions on peter.
This allow matteo to login/logout BUT not to change its own data (password, email..)
You have to manually fix those acls to have that feature to work.

To reproduce:

1) http://demo.magnolia-cms.com (superuser/superuser)
2) Security app
3) Clone "peter": it will create a spare user "peter0"
4) Logout
5) Login with peter0/peter
6) Try to change your profile.
— RED BAR! Error, because you do not have permissions on your user node.

It does not happen if you create a new user from scratch. Just cloning/renaming/import..