The certificate for omega.contacts.msn.com could not be validated. The certificate chain presented is invalid.

It looks like the MSN certificate has expired for pidgin.Remove the file omega.contacts.msn.com. present under /root/.purple/certificates/x509/tls_peers/ or /home/username/.purple/certificates/x509/tls_peers/

Download a new certificate from http://files.andreineculau.com/projects/pidgin/omega.contacts.msn.com.txt. Once it has been downloaded, move it as omega.contacts.msn.com from omega.contacts.msn.com.txt.

Restart Pidgin, it should work properly.