Unfortunately we've been having a persistent issue with new user signups this year, with new registrations getting an "undefined constant error" when they try to submit their information. I've been in contact with Wyrd, one of our back-end admins, to try and resolve this issue, but it seems to be a tricky one so far and not much progress has been made. I'm afraid I've neglected to publically post about it until now, which was inadvisable on my part as I think it has caused a bit of confusion.
I'll be sure to let everyone know when it is fixed, and I'm very sorry if you're a new user reading this and are currently unable to sign up to the forum.