I feel very fortunate that I had success today on my first try for my Madrid to Barcelona train tickets. I also success my first time last year. I know the FAQ is available for folks, but I thought I'd give you a run down of what I did today. Of course, we know the system is screwy and you may go through the same steps and get nowhere, but perhaps there's a step in here that differs a bit from what you have tried.
1. I used a card issued by CitiCards (same issuer as last year, but different card number as mine had been replaced).
2. I called my credit card company ahead of time to alert it of the purchase.
3. I registered as a user on the renfe site (I did not do so last year).
4. I used the Spanish language site. I think this is fairly easy to do (I know very minimal Spanish). If you are concerned, look at the FAQ ahead of time or have google translator open too (www.translate.google.com). I just entered information as it was requested.
5. After entering my credit card information, I was redirected temporarily to a mastercard verification site. I did not have to enter any additional information. It was just checking the card. My card had been through this site for a previous purchase a few weeks ago, which may have facilitated things this time. (That time, I was asked for some verifying information).
6. Ta da. My ticket was purchased. The whole thing took less than 5 minutes.
Again, I don't know why I was successful and so many of you have such difficulty. Check my steps carefully and see if you differ in any way. Otherwise, I think all we can say is what has been said before: Keep trying and best of luck!
And many thanks to Benny for the wonderful FAQ!