 There are several possible causes for this error. First, you can get this error if your username (and password, if you are using database instead of operating system authentication) in the secondary database are not identical to your username (and password) in the primary database. Second, you can get this error if your username in the secondary database is invalid (has not been created). Third, you can get this error if the username/password combination specified in the connect string of the database link definition is invalid (either not created or has an invalid password).