Referring someone absolutely works while being subbed.
You get the friends bundle, free server transfer and 7 days added to your sub as well.
It will work if you are currently subbed, you just can't use another referral link until after you have been unsubbed for 90 days.
Did you maybe use refer a friend when you first started that acocunt (although there seems to be a timeout on that, or was, because I did the old refer a friend when I started the game, when you had to enter their email to get it, and was able to click someone else's referral link this year, but I started in August 2012, and have been continuously subbed since then).
Otherwise, it seems the system may think you have already used someone's link.
