Running Chef 12.5
CentOS Linux release 7.2.1511 (Core)
I created a new Chef user with these commands:
chef-server-ctl user-create test firstname lastname firstname.lastname@example.org 'Passw0rd1!'
chef-server-ctl org-user-add myorg test
If a user has lost their password you can change it via command line on the server with an admin user.
chef-server-ctl password test
This will prompt you to enter and confirm a new password. Give this to your user and let them manage their own key and password.