Het certificaat verloopt (ingebouwd in Office 2019) en heeft niets met te zijn verbonden met internet te maken.

Certificate expiry mechanism
Microsoft's administrator documentation states that "Microsoft 365 apps use a digital certificate to validate licensing. The certificate currently in use expires on July 13, 2026. Apps that are updated to the minimum required versions already include the renewed certificate and continue to function normally. Apps on older versions enter reduced functionality mode after the certificate expires." The minimum required builds are version 16.83 on macOS and version 2.93 on iOS, & those builds in turn require macOS 12 (Monterey) or later, or iOS 17.0 or later.

Office 2019 has no fix. The product line is bounded by a hard build cap below the 16.83 threshold, and Microsoft's own support documentation states the issue "cannot be resolved by updating or reinstalling Office 2019 for Mac." Office 2021 for Mac, by contrast, is still receiving updates through its October 13, 2026 retirement date & can reach 16.83 on supported macOS versions.