Answer
Two configuration areas are the most common cause of this: the Load Balancer's routing/port configuration (per IFS's High Availability Load Balancer documentation, covering how traffic and ports need to be routed across the cluster nodes), and the main_config.json file's HA-specific settings — specifically that System URL, Linux Host, and the nodes array are all filled in correctly for a multi-node setup. A 'default backend - 404' at the landing page typically means traffic is reaching the load balancer but isn't being routed to a working backend, which points back to one of these two configuration areas rather than an application-level problem.