Please make sure that you are using an Oracle 9i or higher Oracle client when you are connecting to your database with SQL Navigator. Here are the steps to select your Oracle client version in SQL Navigator.
1. Launch SQL Navigator and expand the Oracle Home/Client field.
2. Select the newest Oracle client version you have installed.
3. Enable the 'Set as default SQL Navigator Home/Client' option.
4. Proceed to connect to your database and retest the problem.