There are several potential root causes for a complete failure to access any websites. If SSL deep inspection is enabled, but the FortiGate's CA certificate is not trusted by client browsers, all HTTPS connections will fail with a certificate error, effectively blocking most of the modern web. A fundamental network issue, such as an unreachable DNS server, will prevent browsers from resolving domain names to IP addresses, thus blocking access. Lastly, if the FortiGuard Web Filtering license expires, the FortiGate's default behavior is to block all web traffic that is subject to a web filter profile, leading to a complete outage for users.

A. A cleared web filter cache would cause initial slowness as websites are re-categorized, but it would not result in a persistent block of all sites.

C. The webfilter-force-off setting completely disables the web filter service, which would allow access, not block it. This is the opposite of the observed behavior.

1. FortiOS 7.6.0 Administration Guide, Page 958, "SSL/TLS inspection": "For deep inspection, you must install a CA certificate on the browser of each user whose traffic is to be inspected... If the certificate is not installed, the browser will display certificate warnings".

2. FortiOS 7.6.0 Administration Guide, Page 1018, "Web Filter": "If the FortiGuard Web Filter license expires, you can configure the FortiGate to either allow or block traffic. By default, traffic is blocked."

3. FortiOS 7.6.0 Troubleshooting Guide, Page 10, "Basic network troubleshooting": "Check DNS settings... If the FortiGate cannot resolve domain names, many features will not work, including web filter...".