I bought 2 expansions.. one for my main account via the web page, everything went fine, I applied the code on the web page and received my items.
A day later I purchased it for my alt account and got the error with the code (and before you ask, no I was not trying to apply the code to my main account).
So it's definitely something strange going on, that it should work fine for one account, but not on another.
I hope the fix comes through soon.