Access a VM in the VPC through SSH to access the backend VM directly. If the request is successful from the VM, increase the quantity of backends.

Delete the load balancer and backend services. Create a new Passthrough Network Load Balancer. Configure a failover group of VMs for the backend.

Validate the health of the backend service. Enable logging for the backend service and identify the error response in Cloud Logging. Review the statusDetails log field.

Validate the health of the backend service. Disable any Cloud Armor policies on the backend service, and identify any error response in Cloud Logging. Review the statusDetails log field.