@Sebastian-Roth We were able to resolve the issue. The machine has 5 NICs. The NIC we were actually using to network boot with was registered with FOG, but for some reason FOG was reading one of the other NICs during start. After registering all 5 MACs, fog worked successfully.
Re: the HP Elite X2, yes, 2 MACs on one ethernet adapter. Adapter is HP P/N: 829834-001. The second MAC only shows up when network booting - no other time.