A user is unable to log into their account and receives the error message, "Account is locked due to excess violations".
Accounts can be locked out after x amount of consecutive failed authentication attempts.
Account lockout and violation handling are configured within Defender Security Policies.
More information can be found within the Managing Defender Security Policies section of the Administration Guide.
The violation count can be reset therefore unlocking the account by opening Active Directory Users and Computers on a server which has the Defender Administration Console installed.
Once there, open the user's properties and navigate to the Defender tab. The Reset button on the Defender tab allows an administrator to reset the Violation Count which should then unlock the account and allow for authentication.