I had the same problem earlier, tried the system time, MTU size, have no proxys. Nothing worked. However I uninstalled and reinstalled, still nothing, then uninstalled again, cleaned the registry and reinstalled. I also run Skype and stopped it using ports 80 and 443. In hindsight I should have tested after each change but whatever I did it worked. So it was either a registry bug or a conflict with another messenging application like Skype. Hope this might prove useful to someone.
Might have been a bit premature there. Problem arose again today. However it connected on it's third attempt so maybe the problem is with the .net service and microsoft themselves.
