The -605 error should be rare. It means that you tried to edit a task that no longer exists. Maybe you had Toodledo open in two windows and you deleted a task in one window and then tried to edit it in the other window? Or maybe the subtask you marked completed had already been deleted because you checked off the parent right before? Or maybe it was an optional subtask that had been removed because the date had passed, but you hadn't refreshed the list so it was still being displayed in the browser.