We are over some tests at last. As you suggested I did the following:
used a total dumb “link” between the pc and the network (hub): no change, still no boot properly
I used undionly.kkpxe and ipxe.pxe one by one. unfortunatelly no change.
Still no change, that means this hardware has somehow problems. Any other suggestion maybe? The problem is that we will surely get such hw, so it would be good to have a workaround somehow. Ah, one more thing. ipxe.pxe file gave a little difference. It said during boot that it boots from SAN device. Does it changes a thing?