Check your emails, probably already registered and now requires you to log in, it did that to me
I have done, i dont have any other emails from the FA and tried logging back in again, but it just keeps going to the page where you enter the promotional code! May just email them later and ask them to help
I have has exactly the same problem, been trying for 3 days and each time (and it takes a while) it goes straight back to the log in page. Even tempted to pay the £50 to see if that makes any difference!

I have a code lads to cancel out the £50, won't post it (kopites lingering) but pm me.
Then into the underworld of the tout !