A Windows Administrator should review the account located within the LOGIN object of the Job giving the above error and unlock the account.
An account lock can result from too many attempts to log in with bad credentials, an administrator manually locking in the account, or even a set expiration date for the account being triggered.
If the account is not locked and you are still receiving this error message a Windows Administrator should review the Windows event log of the machine which the agent is installed. They may find an error of NULL SID event 4625 within the Windows event log in which case the Windows Administrator may be experiencing the below Windows related issue:
NULL SID Security Log Event ID 4625 when attempting logon to 2008 R2 Remote Desktop Session Host