The error seems occurs when the application tries to get the SQL Handle of your SQL, but I cannot reproduce this error here.
If you have Toad for Oracle installed in your environment, you can run the application with the SQL Monitor of Toad for Oracle, and send us back the SQL logged in the SQL Monitor. We would like to know which SQL cause the problem in your environment.
Also, send us back the log file of the Quest SQL Optimizer for Oracle by the steps mentioned before.
You can try getting plan for few more SQL in the Plan Control session, and let me know if this error appears for the reported SQL only or not.