I use the open-source Pidgin IM client, and came home to discover it had stopped connecting to my MSN Messenger account with the error “Unable to retrieve MSN address book.”
Turns out that Microsoft have blocked the version of the protocol Pidgin’s MSN plugin uses to communicate with their servers. While this is bound to be fixed in an updated version of Pidgin soon, I got impatient and fixed it myself.
To sort this error out, simply install the MSN Pecan plugin and restart Pidgin. If you edit your accounts, you’ll see a new option for account type – WLM. Choose that instead of MSN and it’ll start working again.
MSN Pecan is available for Windows, Linux, and MacOS and is fully open source – albeit unsupported by the main Pidgin developers.
EDIT 20090119: The problem can also now be resolved by upgrading to Pidgin 2.5.4 or later, saving you having to install a third-party plugin.