Not right away after getting the AOR. Been trying to register last night as soon as I found out about the AOR email from IRCC. I kept getting an error even with correct UCI and App No. details. Just few minutes ago, prior to giving you this response, I tried the same registration process and voila, I'm in! Of course, my application status are all still "In Progress"
![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
So my advice is to give it maybe 12 hours or so (from receipt of AOR) to register and be able to check the status of your application.