by Ken Task.
Check: Site Admin menu -> Courses -> Backups -> General Backup Defaults form.
IF 'include users' is un-checked and the 'lock' box (way to the right on the same line) is also checked, then when one gets to the initial page for backing up, you'll see a red X in the option for include users ... with a icon which, when one hoovers over it, tells you the option is locked in position and one cannot select it in the initial backup form.
Looks like:
If it's not that setting then something is messed up but couldn't begin to guess what right now.
'spirit of sharing', Ken