Web banking uses SSL to set up a secure page to work on. Browsers compare certificates to a list of established providers for validation. Since certificates are commercial products, they have expiration dates. This seems to be the problem. If you have a chance to accept it anyway you can try that but it's probably a bad idea. I'd call the bank to find out what's going on.

A certificate is a digital document that guarantees that the issuer has verified the identity of the purchaser. It also contains code to set up an encrypted link for the transaction. My ISP issued it's own certificate and Opera didn't like it when I tried to pay my bill on line.