This is your kernel cmdline:

Command line: BOOT_IMAGE=vmlinuz-4.19.0-6-amd64 
initrd=initrd.img-4.19.0-6-amd64 ip=dhcp  root=/srv/fai/nfsroot:vers=3 rootovl  
FAI_FLAGS=verbose,sshd,createvt,reboot 
FAI_CONFIG_SRC=nfs://faiserver/srv/fai/config FAI_ACTION=install rd.debug

I guess the problem is root=/srv/fai/nfsroot. You hit this bug because
your dhcp server is not the FAI install server.
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=731244

The fix is easy, just add the IP of your install server like this
root=11.22.33.44:/srv/fai/nfsroot....
-- 
regards Thomas

Antwort per Email an