Error Source
Keep in mind that the error message is coming directly from the Hubitat OAuth Servers.
Hub is not responding, please verify it is running and connected to the internet.
That's why we added the blue info box to help clarify and also suggest rebooting the hub as it historically has fixed common issues when the Hubitat OAuth Servers or Hubitat Cloud Relay can't talk to your hub.
Inbound Communication
Also keep in mind that these kind of errors may be limited to just inbound communication to your hub from an external service.
So while testing your hub's ability to ping common global IPs is a reasonable first troubleshooting step, it does not help test the main issue which you appear to be experiencing which is external resources being able to communicate with your hub (eg. via Hubitat OAuth Servers + Hubitat Cloud Relay).
To Larry Kahn's point, the hub establishes a special dedicated connection for messaging and then the Hubitat servers use that existing connection to communicate back with the hub. So if that special connection has issues, then services from outside of your network, including the Hubitat OAuth servers, IFTTT, Google Home, and SharpTools, won't be able to communicate with your hub.
