I found that after installing the latest NOD32, Messenger refused to connect and gave an error code 80072745. The strangest thing is once I uninstall NOD32 and install another anti-virus software, everything worked fine. Also the problem only occurs with certain network connections (e.g. at office, some WiFi points). I think there is some sort of incompatibility issue with the latest NOD32 which may be related to firewall on a network depending on the network provider's setting.