The reason would be due to the SSO page seeing one IP, but the actual server see you as having a different one. The SSO page sees one IP registered to AT&T, but the IP seen by the server seem to be owned by Performive.
The way it works is that basically a key is generated and locked to the IP that is seen on the SSO page and then sent to the actual server (x14 for you). Only be valid when key (stored in a session cookie) AND IP matches what was sent to the server are you able to login through the SSO page, this is a security measure to prevent the key to be hijacked.
Make sure you do not use any kind of VPN or proxy services when connecting to the account.