It is hard to see what is the website that you're having issues with without telling us the exact link to the site. There are many potential issues:

• The certificate was expired not long before you visited the site and the owner has not yet bothered to pay for new/updated certificate.
• The certificate was self signed and it was not authorised by the widely accepted certificate authorities. This is somewhat fairly common.
• The certificate was revoked probably because the certificate authority/authorities was rejected. Like that recent incident with google.com domain hijacking.
• The certificate was probably issued by some dynamically generated site that does not go through the proper certificate authorities. In other words, it could be a Man-In-The-Middle attack.

You maybe secure if you add the exception but again it all depends on your past experience/s with this site (if any) and other potential sources of warnings telling you that site cannot be trusted.

Also I do not believe this is specifically N900 related, if you visit the exact same site with any other device, say a computer you may most likely encounter the same instance if the computer in this instance was updated recently.