Ubuntu 18.04.4 kickstart installation problem

I'm trying to install Server 18.04.4 (ubuntu-18.04.4-server-amd64.iso) via Kickstart. My cdrom:/ks.cfg looks like this:

lang en_US
keyboard de
mouse
timezone Europe/Berlin
rootpw --iscrypted [.hash..]
user bcom --fullname="bcom" --iscrypted --password=[.hash..]
reboot
text
install
cdrom
bootloader --location=mbr
zerombr yes
clearpart --all --initlabel
part swap --size 2048
part /boot --fstype ext4 --size 512 --asprimary
part / --fstype ext4 --size 1 --grow --asprimary
auth --useshadow --enablemd5
network --bootproto=static --ip={{ vm_ip }} --netmask={{ vm_netmask }} --gateway={{ vm_gateway }} --nameserver={{ vm_nameserver }} --hostname={{ vm_hostname }} --device=link
firewall --disabled
skipx

My cdrom:/isolinux/txt.cfg:

default install label install menu label ^Install Server kernel /install/vmlinuz append file=/cdrom/preseed/ubuntu-server.seed vga=788 initrd=/install/initrd.gz ks=cdrom:/ks.cfg quiet ---

The installation at a certain point is interrupted. The following error appears in the console:

An error was returned while trying to install the kernel into the target system. Kernel package: 'linux-generic'.

Digging into the syslog I found more details:

The following packages have unmet dependencies:
linux-generic : Depends: linux-image-generic (=4.15.0.76.78) but it is not going to be installed
Unable to correct problems, you have held broken packages.

The automated Ubuntu 18.04.3 installation is running fine with the same Kickstart config from above. Also a manual install, klicking through the installer, from the ubuntu-18.04.4-server ISO is working.

Any advices or ideas from your side?

Thanks in advance

EDIT:Looks like the problem is not the kickstart configuration, but how I am repacking the ISO. As I said above, the following command makes the ISO for 18.04.3 working as intended. But when I'm trying to run the same command on a 18.04.4 image it doesn't work anymore ("An error was returned while trying to install the kernel into the target system. Kernel package: 'linux-generic'").

mkisofs -D -r -V 'ATTENDLESS_UBUNTU' -cache-inodes -J -l -b isolinux/isolinux.bin -c isolinux/boot.cat -no-emul-boot -boot-load-size 4 -boot-info-table -o image.iso ./iso/ 2>/dev/null

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