Emma *1
but 2 cents more ....
If the course is still available on the system from which you are attempting Xfer of the course, un-enroll Admin level/Teachers from course - might have to un-enroll students as well.
Make a backup; that way and then attempt restore.
There is yet another way ... IF the server no longer online .... not recommended way to do things but when in a pinch ...
Un-archive the backup ... it's a tar.gz.
Edit the users.xml file and remove conflicting accounts ... admin user/teacher - maybe even students.
Also, edit moodle_backup.xml ... cheating with these edits ... in there, one will find things like version of Moodle. One might be able to fool the new server ... depending.
Re-archive ... such that the edited users.xml is now part of the backup. Then attempt restore.
As long as the backup didn't include any special addon that might be incompatible, one *might* be able to restore.
Have I done this before? Yes. Maybe I got lucky ... but like I said ... in a pinch ...
'spirit of sharing', Ken