Try going to HKLM\Software\Microsoft\Windows NT\CurrentVersion\Schedule\TaskCache\Tasks
do a search for the name of your task and delete the key.
Also look under HKLM\Software\Microsoft\Windows NT\CurrentVersion\Schedule\TaskCache\Tree
If you see a tree with your task name delete it also
Open task scheduler and your error should be gone.
To solve the problem permanently I disabled the Network access: Do not allow storage of credentials or .NET Passports for network authentification from Local Security Policies >
Also check the C:\Windows\Tasks tree on the system drive for more remnants.
Hopethis helps G1