Das Zertifikat scheint sich auf Seiten von MSN geändert zu haben. Folgt man dem Link von jabiko, dann findet man folgendes Workaround:
Go to pidgin's Tools->Certificates to
remove the old certificate. Don't
close this window yet. (You can, but
it is easier to not)
With your browser, go to
https://omega.contacts.msn.com/. It
will give you "Directory Listing
Denied"
With Firefox, if you click on the lock
in the lower right corner, you get a
dialog box, where you can click on
View Certificate. On its Details tab,
you can export the certificate to a
file.
Now, back on the pidgin
Tools->Certificates dialog, you can
add this certificate, and all is well.
Beim mir hats funktioniert, Pidgin verbindet wieder zu MSN.