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.
Ensure that you have specified a password for this account. For more information, see Secure the Distributor.