It does not allow me to register because the email is already in the database!
It is possible that with the new email address you are trying to fill in, is already used for a registration and this is the reason why the site does not allow you to use it. There can be only one registration on the site for each email. If you do not remember the login details of the registration made with the email you fill in, you can retrieve the data by using "Forgotten Password".