We got this reply from FileMaker Engineering team: Please use the command "fmsadmin start httpserver" on Terminal, it will fix the problem.
We ran it and it worked - how in the heck were we supposed to know this, where is this info published. We also got this tidbit of info:
It sounds like you are running into the issue described here:
https://support.claris.com/s/answerview?anum=000035352&language=en_US
This issue happens on macOS only, where httpd sometimes fails to start after rebooting the OS. Any time you reboot macOS, we recommend checking if httpd is running or not with this command:
If httpd is running, then there should be multiple httpd processes listed. If it is not running, it can be started with the command provided.
More unpublished information.