Running kernel does not match the kernel modules inside the nfsroot

Hi all!
I really hope you can help me on this because I am pretty lost on what do
do to make it work..

So, I am installing fai 5.10.3~buster1 on a debian stretch (not sure if it
is supported like this but...)
I mostly want to install a debian buster os on a baremetal, right now, just
a basic install.
fai.conf looks like:
LOGUSER=fai
FAI_LOGPROTO=ssh
SERVER=192.168.44.34
FAI_CONFIG_SRC=nfs://$SERVER/srv/fai/config

my /etc/fai/apt/sources.list file (I have changed bullseye for buster since
I only need buster)
cat /etc/fai/apt/sources.list
deb http://deb.debian.org/debian buster main contrib non-free
deb http://deb.debian.org/debian-security buster/updates main contrib
non-free
#deb http://deb.debian.org/debian buster-backports main contrib non-free

# from bullseye on you need
#deb http://deb.debian.org/debian-security bullseye-security main contrib
non-free

# repository that may contain newer fai packages for buster
deb http://fai-project.org/download buster koeln

The content of /etc/apt/sources.list.d/fai.list :
cat /etc/apt/sources.list.d/fai.list
deb http://fai-project.org/download stretch koeln
#deb https://fai-project.org/download buster koeln
#deb http://fai-project.org/download buster koeln

I use the NFROOT config from
https://github.com/faiproject/fai/commits/master/conf/NFSROOT but I changed
bullseye to buster.

Then I have run the fai-setup -v -f .
I have to change a line in /usr/sbin/fai-make-nfsroot
ROOTCMD="unshare --pid --fork --mount-proc chroot $NFSROOT"
because --kill-child is not available as an option in the strech unshare
binary

So far so good, at least no errors at that point.
I have created a host and setup the dhcp
fai-chboot -I -F -v -u nfs://192.168.44.34/srv/fai/config hp2testing
the file is created
So for what I understand, at that pointI should be able to boot and have a
buster installed.
The client boot the pxe image but at the end I got this:

Fully Automatic Installation - FAI
5.18.3-busterl (c) 1999-2821 Thomas Lange Clange9cs.uni-koeln.de>
inux 4.19.0-9-amd64 x86_64 GMU/Linux ernel parameters:
BOOT_IMAGE=vmlinuz-4.19.8-9-amd64 initrdminitrd.img-4.19.8-9- .md64 irdhcp
root=10.199.44.34:/srv/fai/nfsroot:vcrs=3 rootovl FAI_FLAGS=menu
initial.verbose.sshd.createvt FAI_COMFIG_SRC=nfs://
10.199.44.34/srv/fai/config Al_ACTIO11=install 'coding /tmp/fai/boot.log
Al_FLAGS: menu initial verbose sshd createvt getting SERVER=10.199.44.34.
Value extracted from FAI_COMFIG_SRC. Can't connect to monserver on
18.199.44.34 port 4711. Monitoring disabled. RROR: The running kernel does
not match the kernel modules inside the nfsroot. RROR: Kernel modules
directory /11b/modules/4.19.0-9-amd64 not available. Only found:
lib/modules/4.19.8-19-amd64
rror in task confdir. Code: 790 aceback: task error task_confdir task
!start main LOGUSER is undefined. Not saving log files to remote. FATAL
ERROR. Installation stopped. root hp2testing:/* _

(sorry if the output is ugly, I use an online image to text converter...)

So at the end the kernel used to boot 4.19.0-9 is not the correct one found
in the nfs server 4.19.8-19, but why?
What step have I missed to have this error?
If you can guide me on how to troubleshoot this I will really appreciate it.
thanks!