by Emma Richardson.
I am thinking that the act of deleting the users first might be causing some database issues when you reset (database is trying to delete assignment entries for now non-existent users). I would try resetting the course first and making sure that you select everything to reset.
Another thought - When you add the new users, are you allowing duplicate email addresses? And do you allow the use of email to log in? Could it be that somehow the new users are actually coming in under the old user account? Or do you use default usernames like Student1, Student2?