While browsing a website, a staff member received a message that the website could not be trusted. Shortly afterward, several other colleagues reported the same issue across numerous other websites. R

A bad antivirus signature update might block specific legitimate sites or interfere with applications, but it is less likely to universally cause "website not trusted" errors across numerous disparate websites for multiple users.

A misconfigured router or firewall within the corporate network could be blocking or improperly routing secure HTTPS traffic, causing browsers to report trust issues due to inability to establish secure connections or certificate validation failures. Since only internal users are affected and remote users are not, the issue points to an internal network device interfering with web access.

If an upstream internet service provider was experiencing issues, both internal and remote users would likely be affected, which contradicts the observation that remote users had no problems.

While incorrect time or date on client machines can cause certificate validation errors, it is unlikely that numerous machines would simultaneously have their time/date drastically desynchronized to cause a sudden, widespread issue across an organization.