When making a course manual backup user data is locked by dependencies if not checked by default in general backup preferences even if checked in backup form.
To reproduce:
(as manager)

• Go to General backup defaults:
• Uncheck backup | backup_general_users
• Leave locked unchecked
• Go to a course backup
• Check Include enrolled users
• Click next and see user data is locked by dependencies.

If backup | backup_general_users is checked in general backup preferences backing up user data works as expected.