The important thing is, in my case, that I did not select any execution date (NULL attribute) within the workflow generating the task, so the task is on "standby" waiting for a date and time somewhat in the future in order to execute. If you go ahead then, edit the task and select a future time, it will fail with said error.
But you should be able to repro the issue too, by simply generating a manual task and then try to re-run it, after it completed (or failed).
Another missing feature is the possibility to simply "Run" a task (e.g. scheduled in the past).
The idea is, that you can use a task in order to prepare the parameters for a workflow to reverse a procedure, e.g. to delete a certain VM again, without the need to browse and select the name within the vCenter tree. So the initial workflow creates a VM and generates the task to destroy the VM again and passes the name of the created VM, but the task - in my case - is not scheduled, so the operator simply should be able to either schedule it himself or to run it, when the VM is no longer needed.
