Recently users where getting this error message:
“Revocation information for the security certificate for this site is not available. Do you wish to proceed?”
when they accessed a website.
After investigation it turned out that the certificate revocation list distribution point, which is found (in IE anyway), under:
Page properties, certificates button, look on the details tab for CRL distribution point.
Was no longer accessible, due to some interesting firewalls that had been put in place. On opening access to this appropriately, the message no longer appeared, as the revocation information was then available.