New Post: How to update the additional arguments of an existing task?

Finally I found the solution. The above error message was a little bit confusing :-).

The tasks we would like to modify were not located in the root folder but in subdirectories.

So the following code is finally working:

Using ts As New TaskService("YourRemoteServerName")
   Dim task = ts.GetFolder("YourSubdirectoryName").Tasks("YourTaskName")
   If task IsNot Nothing
      Dim action As ExecAction = task.Definition.Actions("YourID")
      ' Dim action = task.Definition.Actions(0)
      action.Arguments = "new argument string"
      task.RegisterChanges()
   End If
End Using