There have been many reports of issues with the online tool - they have been reported in the
xONEx online tool bugs thread
But IIRC your's is the first report of problems with an ex-EZE itinerary (most of the problems reported have been to do with ex-KRT itineraries and/or with the first flight being a codeshare)
I have just tried a very simple itinerary (EZE-DFW-NRT-LHR-EZE)
and it priced up OK and when I completed the (spurious) payment details - it failed with the message "We are having difficulty accepting the credit card details provided. Please validate and try again. This may be due to an invalid credit card type, in which case please provide another one (4613 - 201)"
Can you post your itinerary for us to check?