Scheduled tasks need a password

I set a user up with a second disk to store backups of lots of photos. And set up a SyncToy pairing to make sure the backup disk kept a copy of all the pictures. And then set up a scheduled task so that would do this backup automatically at a certain time each day. But the scheduled task kept failing. It turns out that the user must have a password for scheduled tasks to work. A blank password is not allowed!