Indeed, in fact if I remember correctly the club introduced a voucher scheme the season after the cup final for fans to collect should we go on another mighty cup run. I might have this all mixed up but I'm sure the first one was against Swansea at home, managed by John Toshack at the time, and...
Are they on general sale now? I'm thinking you still need a purchase history and your new account won't have one. The system is clever enough to know that but giving you a simple message about it is beyond its capability.
Try logging in before you go to home tickets. I think where you're clicking isn't actually a link to sign in but more a reminder that you have to sign in.