It appears that, intentionally or not, Kobo has split the contest across servers.
The old URL
http://contest.kobo.com/ will only allow registration and access for the UK and CA.
The "new" URL
https://www.kobo.com/contest/ will only allow registration and access for the US. I'm guessing that this URL should be good for future contests, but if it doesn't work for you, these both work for the current contest.
https://www.kobo.com/contest/game?na...r_dark_stories
https://www.kobo.com/contest?name=after_dark_stories
If you register for the US, you can't use the
http://contest.kobo.com/ URL to play.