When I try to connect from my PC with SQL*Plus to the Oracle server on a different machine, I get the "ORA-12571:TNS:packet writer failure" error. But I can successfully log into Oracle on the same machine from SQL*Plus. I shall be highly obliged if you could kindly give me the solution to this problem.
The ORA-12571 error message is telling you that the network packets could not be transferred for some unknown reason. There are a few common reasons. If you are trying to access the database through a firewall, then the firewall may not be configured correctly to allow the Oracle connections. When you contact the listener on port 1521, the listener hands off your connection to a dedicated server process. You will communicate on a new, random port to that server process. Firewalls can stop communications to this new port. This may be affecting your connection.
Another possible problem could be network related. Have you tried to start a FTP session to that server? If you can FTP to that server just fine, then the network connections to the database server are good.
Another problem occurs when spyware or a personal firewall on the workstation is inadvertently causing problems with your network communications. Removal of the spyware or proper configuration of your personal firewall will resolve these issues.
Additionally, an improper configuration for in your SQLNET.ORA file can be causing problems. Remove the SQLNET.EXPIRE_TIME=0 entry in the SQLNET.ORA file on both your client and your database server.
If none of these suggestions solve your problem, you will have to contact Oracle Support.