Google Cloud Nightscout » Troubleshooting » Connection Failed
You may experience a connection failed error.

That will be an expected error after a reboot.  You are remotely connected to a virtual computer.  When you reboot that computer, you cannot still be connected to it.
Wait 30 seconds;  then, click on retry to reconnect.
You may even experience the error unexpectedly.
Wait 30 seconds;  then, click on retry to reconnect.