I think that this occurred because you deleted the qvw files, correct?
I've also had this problem a while ago. Take a look on this post: http://community.qlik.com/forums/p/26401/101157.aspx#101157
You basically have to:
1. create a dummy qvw file (with the same name as the task's file) in the folder
2. disable the task
3. delete the dummy file you've created.
Hope this helps.