Error MSSQL_ENG018456 is raised whenever a login attempt fails. If the error message includes the account distributor_admin (Login failed for user 'distributor_admin'.), the issue is with an account used by replication. Replication creates a remote server, repl_distributor, which allows communication between the Distributor and Publisher. The login distributor_admin is associated with this remote server and must have a valid password.
In versions prior to MicrosoftSQL Server 2000 Service Pack 3 (SP3), it was possible to specify that this connection was trusted and did not require a password. If you are upgrading from a previous version, you must now specify a password.
Ensure that you have specified a password for this account. For more information, see Securing the Distributor.