This is a recognized bug in version 9.0. When I contacted Support about this, they said it should be fixed in version 10.0 which is scheduled to be released late this year. In the meantime, you need to clean the Repository manually. See the last post on this page for more detailed instructions: http://community.qlik.com/forums/t/25325.aspx. This is a fairly time-consuming process, so keep in mind that it would be a lot simpler to just delete all the tasks by deleting/renaming the QVPR folder entirely.