error/root_only_one_ocurrence

When I try to make a course backup I get this error. There is also a link displayed to a Moodle Doc page that has not yet been written.

Can anyone point me in the right direction?

Moodle 2.7.2 (Build: 20140908)

To clarify things: this is a clone of our server in production. We would like to use it to do course-backups on demand when teachers need a course restore. We use ESX vms now. So this could be sort of a snapshot my colleagues restore directly into a separate vm. But I have to sort this out first.

Thanks a lot, Rosario