What to do when Fonts are replaced by "Boxes with questions marks" inside them

On macOS 10.14 (“Mojave”) there have been reports that fonts in dialog boxes and/or web sites may be getting replaced by boxes with question marks such as:

Open

If this happens, we would first recommend the operating system be upgraded off Mojave to something supported (10.15 and up as of 11-29-2021)

If it’s not possible to upgrade the OS, please go to Applications → Utilities → Service Provider Support. Find the “Flush Font Cache” task, hit the “Flush” button and reboot the computer when that has finished and that should resolve the problem.