I cannot get SyncBack to run under another users account, e.g. Administrator, when using the Scheduler. Why?

Profiles are by default user-specific, as they are stored in user-specific locations by user-name (for security reasons). The location SyncBack will look in for profiles when the Scheduler calls it is dictated by which user is running SyncBack, so if you change the credentials in a Scheduled Task, the Task will run as that user, as will SyncBack, and SyncBack will thus look in that users location for the named profile instead, which may not exist there.

You can share profiles between all users if the profile is stored in a folder all relevant users can access (they must have read and write access to the folder). See this article for more details how to do this, and also to change the logging settings (highly recommended).