ConCen's certificates are never self-signed. They are issued to concen.org by RapidSSL CA and signed by GeoTrust Inc.
If you are presented with a certificate that doesn't match these two important criteria, do not accept the certificate. It is forged! Someone may be trying to intercept your login information.
If these two criteria are correct, you can safely accept the certificate. Once you do so, you will not be bothered again unless the certificate changes. If it does change, make sure it's not a forged certificate!
Please go to the secure site only if you understand how to recognize a forged certificate!