I guess the problem is getting a little bit more clear to me.
My guess is that we all use iTunes giftcards (as some of you already said)
and no credit card linked to our accounts.
So this could be the problem that we all are getting the unkown error issue.
But then again, as we all can read in the terms and conditions of iCloud, it says that a valid credit card is needed for purchasing extra storage for iCloud, "AND" as well is itunes store credit a legit way of purchasing extra storage for iCloud.
So my guess is, they would accept iTunes store credit, but only when you would have a credit card linked into your account.
I hope this isn't true and it is just another problem, but it all points in this direction.
That would be a big fail.