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"

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.