When a user is locked out, use Server.app to reset the user's password to allow them access again. If desired, you can force the user to change their password again after logging in.
OS X Server: How to unlock a locked user account
When a user makes too many unsuccessful attempts to log in to OS X Server, the account will be locked if it meets the limit set by server policy. Follow the included instructions to unlock a locked user and allow them access.
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