I recall I had a similar issue when the new site was created, so go try
www.cardiffcityfc.co.ukChoose 'Buy Online' from the Tickets drop dow menu at the top
Either 'Sign In' or 'Register' (top right)
and understand that this is not the same user name and password as the site at the top of this postYou will also have an option to 'Activate' an existing account, after which you should be able to see your option to renew your existing Season Ticket (hold also be in 'Notifications'
I hope it works but if that fails, I suggest you contact the Ticket Office directly for assistance