MSN messenger (or rather live messenger now,) has updated their authentication certificates among other things a couple times over the last few years. So it wouldn't be surprising that MSN messenger doesn't work anymore. I had to go and download the latest certificates in order to get Pidgin to work a couple months back. Have no idea how one would do something like that for Mac OS 9. I haven't used Yahoo in a long time so I don't know about it.

Making a multi-protocol messenger for Classic Mac would be a pretty neat retro challenge. Starting with one protocol and make it expandable with modules. Oh well. Guess IRC and Jabber are well enough for now.