Can't boot installer off PXE server

[snorfu]

Please excuse if ubiquity is not the appropriate module.

1) 20.04
2) n/a
3) Expected result: 20.04 can be booted just like 18.04 off a PXE server
4)

Scenario:
ubuntu 20.04
I copied all the files from /casper to a directory on a PXE server, the relevant part in pxelinux.cfg is

LABEL u200464lts
KERNEL u200464/casper/vmlinuz
APPEND root=/dev/nfs/ boot=casper netboot=nfs nfsroot=192.168.100.205:/sda5/misc/netboot/u200464 initrd=u200464/casper/initrd --

I boot a machine using PXE, select the abovementioned entry.
screenn output:

Loading vmlinuz...
Loading initrd..
..

boot process continues to

Begin: Running /scripts/casper-premount ... done.
done.
Begin: Trying netboot from 192.168.100.205:/sda5/misc/netboot/u2004
connect: Network is unreachable
NFS over TCP not available from 192.168.100.205

this is repeated until a busybox is shown

All file permissions are set to 444

Using 18.04

This is the relevant part of pxelinux.cfg

LABEL u180464
KERNEL u180464/casper/vmlinuz
APPEND root=/dev/nfs/ boot=casper netboot=nfs nfsroot=192.168.100.205:/sda5/misc/netboot/u180464 initrd=u180464/casper/initrd --

...and everything is working fine, after
Begin: Running /scripts/casper-premount ... done.

the network interface is configured.