Shoutbox

Incase it isn't a bug with StuffPlug's polygamy soft patch, whch seems likely, there was a recent similar problem that was resolved by doing the following.

Tools > options > general > 'sign-in accounts and passwords' > then click change and then delete all accounts, then exit and reopen MSN Messenger.

Personally I can't see why that works, however it did fix this problem for someone else. Since you can add accounts back again with minimum hassle it is worth a try.