The CreateItemTask has been deprecated in the underlying services (REST/SOAP) since BA Server SP1 and is marked as deprecated in the REST/SOAP Reference documents. In SP2, this has been removed and 2.1 works with SP2. I am not sure whether you have any Busness Analyst Server 10 Service Packs yet on your instance.
CreateItemTask used to create an empty item which could not be opened, so if an item is created using this task and another task is executed that tries to save the output as this item, the task will fail since the already created empty item cannot be overriden. Instead, folder items should be created by specifying the output parameters of tasks (e.g. using the outputAnalysisItem parameter for feature layers) through which they are created automatically. Use this approach instead where you use CreateItem.