To clarify, I am talking about "Vger's Module" (the one this thread is about, correct?) not the one that comes with OSCommerce or the other one floating around that is a bit older, though I have been unable to get any of them to work correctly. I can go into the errors I get with the other 2 (default one charges card but gives an Authorize error page to customer - bad; the other gives general error in cart and does not go through at all, similar to this one), but it seemed this one was the one most people had said they could get to work, and it includes the card verification code which is important to us. I'm having no luck, though.
MasterCard and Visa also fail, so it's not just AMEX for us.