Have you ever been able to establish a connection to the database server from this client? If so, what has changed recently that may explain this problem?
This error usually indicates that your connection to the database cannot be established because it does not know where to find the database. Check the tnsnames.ora on the client workstation to ensure that you are using the correct host name or IP address of the database server. Also check that the service name is correct in the tnsnames.ora. Finally, check to see whether there is a default domain identified in the sqlnet.ora. If so, try using the default domain in the connect string. For example, if you are trying to connect to a database called PROD and there is a default domain of .WORLD identified, trying connecting to PROD.WORLD rather than just PROD.
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