Comment 8 for bug 1172171

Just ran into this issue as well. Had to create a new account as my old one has been merged and could not log into UOS. The new account cannot register for UOS because of this exact issue.

From comments in this bug, I gathered that I needed to register in launchpad first (perhaps useful to put this pre-requisite on the UOS front page): https://launchpad.net/sprints/uos-1511

Logging in with new user works registering in Launchpad, but registration does not sync immediately:
You are not registered as attending. You can register your attendance in  Launchpad. If you have recently done so, wait a few minutes and reload this page.

After a few minutes I was able to register.