I've tried to define NLB cluster for two portal machines. The NLB mode is multicast, because there is only one network adapter on every machine. Configuration was OK, reconfigured Web Adapter showed right URL to cluster FQDN, but access to this URL failed - resource not found (404).
Seems, portal doesn't like the method, used Windows to achieve load balancing - creating virtual network interface with mutual MAC address.
Environment: Windows 2012 R2, Portal for ArcGIS 10.4.
Thanks in advance.