No kidding! I have been running into the same problem trying to book an SJ ticket from Stockholm to Copenhagen for days! Neither my Chase United Club visa card, my BoA Mastercard, nor my Fidelity branded Amex card worked. The only card that worked was the Costco Amex card (in that case the issuing bank is Amex itself, unlike the Fidelity branded Amex card which is actually issued by BoA's MBNA division).
Thank you FT for this expert knowledge. Saved me a lot of grief.