sharing a quick fix as I couldn't find easily information on how to get rid of this error. This also prevents from adding/deleting new tasks to the system. In my case it was a orphaned task so it was easy to get rid of this failure
You can check "C:WindowsSystem32Tasks" and get the above XML. as mine was orphaned I went ahead and removed the XML (=task) file from this folder and I could again access "Task Scheduler" MMC or via Powershell
Task Scheduler Schema
https://msdn.microsoft.com/en-us/library/windows/desktop/aa383609(v=vs.85).aspx