FYI, for anyone looking for this in the future. I had this problem too and it started after I had to dispute charges on my credit card when someone made fraudulent Starbucks charges to my card. I'm pretty sure my Starbucks account online got hacked at some point.

Ever since, online topups and auto-topup always fails for me. I end up just refilling whenever I'm at a store with some time. I'll try the Starbucks forums though.