After reboot, the OPNSense appliance stopped servicing the WAN or OPT1 interfaces. The network traffic inside the local network was unaffected.
The logs show the route failure, but they act like the incorrect network switch cable is plugged into the appliance.
Pings on the appliance, menu option #8 or though the Management GUI, result in the following message:
$ ping 188.8.131.52
ping: sendto: Invalid argument
$ drill www.mydomain.com
Error: error sending query: Error creating socket
In the system log:
... arpresolve: can't allocate llinfo for <network ip> on OPT1
If you disable the IDS / IPS service, and reboot, the network will start working.
The IDS or IPS service is causing the outage.
Either leave IDS/IPS disabled. (Not an option for us)
Update the gateway for OPT1, and click the checkbox "Far".