Cannot login to Live Messenger

Months ago I've installed Live Msn on my XP SP2 and used it without any
problem.
Suddenly, about two weeks ago, I couldn't connect any more.
I always get the 80072efd error, even with the firewall down (I feel like
I've tried to change any possibile setting...).
The Solve Problem's procedure says that my gateway is offline but, needless
to say, I can freely surf the net with Explorer... but what is incredibly
surprising is that if I try connecting with the old MSN 4.7 (that I never
disinstalled) I can login and chat!!!

I lurked the internet's forums and tried some fixes for similar problems but
none worked until now... please help!!!