Thanks for your reply, Emma.
In the course table in the database there are two rows. One row corresponds to the definition of the site (for example, the "course" name is, in fact, the site name). It includes all the resources that appear on the front page and are not part of any single course. The other row corresponds to my course. Remember, I am referring to the automated backup whose settings are made under Site Administration, not the manual backup that one can launch under course administration. So, the backup does indeed include non-course objects. And the simple fact that those site-related objects are indeed correctly backed up shows that there is no issue at all with permissions. Anyway, this is not really the issue about which I was asking. I am trying to understand how the retries of the failed backup are managed.