how to diagnose installed FAI server

[e.fernbach at gmx.net]

I installed a FAI server on a fresh Ubuntu 18.04 install following 
https://fai-project.org/fai-guide/

up to the point where it says "At this point, you should have a working
faiserver setup and your clients should boot into FAI and be able to
install one of the examples."

When connecting a client with PXE boot (Realtex PXE B04 D00) it shows
the DHCP ... and a circling I for a while then PXE exits with the "boot
failure" message.

How do I further diagnose this? Can I somehow test if the server tries
to issue an IP? What configuration does the ethernet card of the system
running the FAI server have to have?

Thanks for your consideration