I think you should remove the Asrock MD in webadmin (leaving only the core/hybrid, and the other PC you just tested), reboot everything, and try again.
You have to be reeeaaally patient, especially with these small low power boards - they just take longer. If you think it might be stuck, just leave it, go to bed, leave it overnight and check it in the morning. If it's still stuck, give it a reboot only then, and see what happens.
Or, the motherboard is knackered. Try installing a normal Kubuntu desktop on it and see what happens, see if the NIC works, etc (you'll need to find a hard drive for this obviously).