Цитата из HELPa:
ORA-12203 TNS:unable to connect to destination
Cause: The client is not able to find the desired database.

Actions:

1) Verify that you have entered the net service name you wish to reach correctly.

2) Verify that the net service name ADDRESS parameters in the connect descriptor of your TNSNAMES.ORA file are correct.

3) Verify that your TNSNAMES.ORA file is stored in the correct directory.

4) Verify that the listener on the remote node has started and is running. If not, start the listener by using the Listener Control Utility.

5) If you are connecting from a login box, verify that you are not placing an "@" symbol before your connect net service name.