by Nadav Kavalerchik.
[solution] After some digging...
I found out that the error was a little bit different from the one above, which I was also getting, on Moodle 2.5 but not on Moodle 2.8, in which I was getting: "error/moodle/wrong setting level moodle backup xml file" and the fix is: https://docs.moodle.org/25/en/error/moodle/wrong_setting_level_moodle_backup_xml_file