Hi Daniela,
I have just been attempting to use the Braintree module on my OSC today, and am running into exactly the same problem as you, receiving only the message "The card number was not able to be processed. Please try again and if problems persist, please try another payment method." May I ask how you fixed this, or, could you perhaps list all your module settings, so that I can try it out also?
NB. I am currently running OSC2.3.3, not 2.3.4 - not sure if that is also a factor..
Many thanks,
Nicholas