It seems that your attendee is an account instead of has an account. Since accounts have quite some restrictions your import propably fails due to the fact you are not allowed to change the reference. It would have been better to have an association between attendee and account and indeed the association between attendee and eventlocation.
So double check your security that indeed you are allowed to set the reference and that the role that does the import is allowed to manage other accounts.
Regards,
Ronald