After some hours trying to desperatly find the solution. I finally sent the Boot 2 successfully.

Here´s what you need to do.

First, try to see if you have slimmer cables if the ones you have are not so slim. That will be handy. To determine if they are slim enough, just see if it´s easy to stuck them correctly by just pushing it under the plastic cover in the connectors. The cables I was using weren´t exactly that slim, sometimes they would stick and when they did just a small movement would make them fall.

And once you go to the "Begin XMODEM file transfer" message, just carefully start to play around with the TXd cable (don´t let the other cables move) connected to your calculator. Start to push it or pulling it accordingly until those sybols in the screenshot stop multiplying. Once they do, send it normally.