Me thinks you have a role in the backup file causing the issue + a non-interactive setting?
So do this:
Extract your backup.mbz
tar zxvf backup.mbz
edit moodle_backup.xml file
Change the following:
<name>role_assignments</name>
Value set to 0 ... not 1.
<interactive></interactive>
Set to 1:
<interactive>1</interactive>
re-package the .mbz
tar -czvf backup.mbz *
When restoring and you get to the interactive part, make sure
you do NOT restore roles ... un-check if there is a box for you to decide.
Resorting a course that might have an admin account level in it conflicts
with admin users already in the system.
Might consider creating backups with no roles in future ... especially IF you
have an admin level user in a course. You can always re-assign roles to the course.
BTW, tested this on a 2.8.highest version of Moodle and it failed the first time ... almost right off the bat. The suggested error link didn't mention specifics enough. So I inspected and compared a valid/working .mbz backup to yours.
After making the changes to the moodle_backup.xml file above, restored without issue ... **did un-check option to restore roles on my system** ... for obvious reasons.
'spirit of sharing', Ken