Perhaps it might have had something to do with keeping the window open or not, it seems like a similar issue I had with one of my other accounts.
If the Registration closes before you activate your account, you'll have to wait until the next registration opens before you can activate it, it might have something to do with the system or the way it works. I'm not so sure, but hopefully you'll be able to activate it soon!
I apologise in advance if my post is not very helpful.