Strange Error on Checkout

I have a client who has a customer who gets the following error when they try to checkout:


It lets me log in fine, I go all the way through putting my card number in, click "next", then it always gives me this message:

Message

* There was an error while processing your transaction: 25 An error
occurred during processing. Please try again.



We're using UPS as the shipper and Authorize.net as the cc processor. Other orders go through fine.

Any ideas on how I can solve this?

thanks!