Hi there!
I see you did not find a solution for the problem.
For three days, I've got it too. First it started with Vista, but I've had enough of it, reinstalled XP and thought I'll get rid of this problem too. I didn't.
Now I have WinXP SP3 and WLM. I can sign in with both my addresses at home. I cannot sign in from my work location, but *only* with my primary msn account. Secondary works fine. What's more, if I try Pidgin IM, the primary account works as well, but not with WLM.
I don't have any extra firewalls - only the one from windows and it cannot do any harm. Especially if I'm able to log in with a different msn account. Which I am. WTF?!
It cannot be a computer problem, because I use a laptop and so signing in at home or at work only changes the location, not the computer. WLM installation must be fine. It cannot be company's firewall, because one account works fine ... and the primary too - if I use Pidgin IM.
Has anyone got a clue wtf is causing this?