Advanced SQL Optimization error: The following unhandled error has occurred: Message: Error from Loading Return Request Session Command (4302009)

Toad for Oracle | Advanced SQL Optimization | SQL Optimizer gives the error:

The following unhandled error has occurred:

Exception happens at
Class : TConnPoolXMLCOM
Method : Conv XML
Module : LabOptimizing
Message: Error from Loading Return Request Session Command

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Multiple Oracle Clients installed and the default client is not being chosen or used in SQL Optimizer nor 'tnsping'.

Toad for Oracle is using the default Oracle Client 11g Home as shown in PATH.  However, SQL Optimizer seems to switch to a different Oracle Client Home.  When 'tnsping' is run, it also shows it is using their 10g Client, not the default 11g Client.  The Oracle 11g Client installation does not look complete or may be damaged. 

Re-installing the Oracle 11g Client and ensuring that 'Administrator' installation type (radio button) is selected during the 11g client installation resolved the error produced in SQL Optimizer.