@Sebastian-Roth
Im having the exact same issue as well, with the same NIC (14e4 1686). Im using a thunderbolt -> Ethernet. I have only tested with one macbook air so far (El Cap 10.11.5, early 2015 macbook), and I am getting the no DHCP on net0 on bootup. Running an isc-dhcp server and every aspect of it is working fine other than getting the mac to obtain an address (my one windows machine works perfectly, as usual). I am the network administrator for an exclusively mac environment and I have access to all hardware from a 2003 era Xserve (thats still in production) to brand new 2017/2018 iMacs and everything in between. Let me know how/if I can help and id be glad to. For reference I did select an SSID on bootup once and I was able to obtain DHCP from net1 (wifi card), but not only do I not want to image over wifi, I am currently running the fog server and its own DHCP server on an isolated network to rule out any firewalls/weirdness. My setup consists of exactly 3 cables, my secondary NIC on proxmox is configured to be an isolated 172 network, I have that patched directly into an 8 port switch on my desk, and I have a direct run from the firewall going into the same switch, with my mac client being the 3rd cable in the switch. Im pretty confident we can rule out any networking issues, though id be more than happy to run any tests you think could help.

Sebastian