Greatly thanks for reporting this!
There was a big problem there preventing every new user from registering an account. I committed a revision to the website's scripts about a week ago to prevent possible web attacks in the form of MySQL injection. This security hole was pointed out by Zieliński
who managed to overwrite portions of the database in a controlled manner for testing the security level. Since I fixed the hole, I happened to leave an extra copy-pasted line in the wrong position, preventing some variables to pass thru the scripts, such as the user's email address.
In fact, I've been wondering all the last week why no new users were registering to Greatturn.
Well, problem fixed now. Try it again!