Since the CIC requires payment online now, and the old receipt form (IMM 5401) method is no longer available, I presume only those who have used that old form can make the site work...
http://www.cic.gc.ca/english/helpcentre/answer.asp?qnum=1246&top=4
I paid online for our application just last week and printed the receipt, but neither the receipt number and reference number work on the "check your application status" page. Seems like I will have to wait for the AOR to receive the actual UCI number.