by Ken Task.
Site1 is what version of Moodle?
A role conflict could apply to roles other than admin level ... example: the backup has a user as Manager or Teacher and in Site2 that user has a role of student. That would be an escalation of access and something not desired.
It's not just the admin users - any whose role in backup would escalate access level in new course/site.
So if the users already exist in Site2, the one thing you haven't tried is a no user backup of the course, restored, then assigning users and roles in the newly restored course.
'SoS', Ken