I am no travel agent, much less a guru, but after buying a ton of South America - Europe round trips, on very similar AA fares, I consider myself a minor expert on the matter.

IMO, the basic problem with your itinerary is that by going to LON, and merely connecting in MAD, Sabre is, in efect, pricing it as an EZE-LON fare, not EZE-MAD, and since LON is a higher fare point from EZE the result is a no-no, because that fare is not available to LON.

You have 3 options:

1. Find a creative TA to ticket this for you and pay the fee.
2. Forget MAD and price it on aa.com to LON, where it will cost ~$1650
3. Forget LON and price it on aa.com to MAD, where it will cost $1491

There is a 4th option, calling AA and talking to an agent, but unless your itineray is a plain vanilla EZE-MIA/DFW-Europe it will all end up being an exercise in frustration and a total waste of time. Most AA agents don't have a clue about these fares, and the first thing they say, by reflex, is "you can't do that"; from then on it is like talking to Adrian's Wall.

What you want to do is doable through aa.com, with a little tweaking here and there and some compromises on your part.

Finally, a word about ITA. It is great, but the more creative ITA itineraries just cannot be ticketed by AA, simply because Sabre is not perfect; fact of life, and one I have learned to live with.