Thanks for your suggestions, jboyton.
I tried to add a delay, like you suggested,, but with no better result.
In fact, by trying higher values, i noticed that the threshold to receive the second character is around 1010 microseconds. Given that a character takes about 1041 microseconds at that speed, maybe the switch to rx mode is really too slow, but then i have no idea why it works at 57600bd.
Also, when i invert the disableTx() and listen() lines, the code works, but that doesn't sound like a good solution to me.
I do not have a oscilloscope, nor a second arduino right now, but i will try to use xoscope to monitor the line.