|Hi again, everybody|
After calling telefonica, 3 weeks ago, the problem seemed to be solved, but the following day the problem came back. I called Telefonica again to complain about it, and they couldn't give me any positive answer, just a "we've no idea, we can't do anything about that ...".
So, now I've followed the solution given by Richard Parcent, and I've replaced my DNS server IP's by the Googles one's (220.127.116.11 and 18.104.22.168 - not just added, but removing the telefonica one's - just in case, take note of them, in case you want to reestablish the previous configuration).
So, it seemed that it was a compatibility problem between the Mac access to telefonica DNS service. Thanks Richard!