Hi,
this is a bug in 9.6, it was reproduceable by LANDESK Support and is commited as defect (I was told by support). You can copy and paste the task on an additional console as a workaround, that works fine. On the core, you can still use the drag and drop feature.
But you have to be careful if it is a repair task. If you copy the task and delete the original one, the copied one is not usable anymore, no right click is possible, the task is absolutly usless. You have to delete this task through the database. This is reported to support too.
Hope it helps.
Kind regards
Christian