I noticed recently that the Pidgin 2.7.5 messenger  had problems connecting to MSN with a certificate error about omega.contacts.msn.com. Fortunately I’m neither the first to have that problem, nor to find a solution .
MSN apparently replaced their certificate, which now needs to be updated with Pidgin.
1. point your browser to https://omega.contacts.msn.com/
2. get the certificate and export to a file
3. name the file omega.contacts.msn.com
4. copy the file to %AppData%\purple\certificates\x509\tls_peers and replace
5. restart pidgin