20.04.1 autoinstaller failing

I'm trying to create an autoinstalling image from 20.04.1 that I can distribute on USB or DVD media. To obtain a valid yml file as a starting point, I installed 20.04 manually and then copied the autoinstaller file from iirc /var/log/installer/ from the virtual test machine to my workstation.
Erasing the already known bugs (like adding a "version: 1" before autoinstall, changing toggle: null to toggle: '', adding a secondary layer "network:"...) I fed this file back into the 20.04.1 live server image:
<pre>
version: 1
autoinstall:
  version: 1
  locale: en_US
  keyboard:
    layout: de
    toggle: ''
    variant: ''
  apt:
    geoip: true
    preserve_sources_list: false
    primary:
      - arches:
          - amd64
          - i386
        uri: 'http://de.archive.ubuntu.com/ubuntu'
      - arches:
          - default
        uri: 'http://ports.ubuntu.com/ubuntu-ports'
  identity:
    hostname: [hostname]
    password: >-
      [passwordkey generated by former install]
    realname: standard
    username: [username]
  network:
    network:
      version: 2
      ethernets:
        ens160:
          addresses:
            - [static_ip/netmask]
          gateway4: [gateway_ip]
          nameservers:
            - 1.1.1.1
          search:
            - [searchdomain]
  ssh:
    allow-pw: true
    authorized-keys: []
    install-server: true
  storage:
    config:
      - ptable: gpt
        path: /dev/sda
        wipe: superblock
        preserve: false
        name: ''
        grub_device: true
        type: disk
        id: disk-sda
      - device: disk-sda
        size: 1048576
        flag: bios_grub
        number: 1
        preserve: false
        type: partition
        id: partition-0
      - device: disk-sda
        size: 1073741824
        wipe: superblock
        flag: ''
        number: 2
        preserve: false
        type: partition
        id: partition-1
      - fstype: ext4
        volume: partition-1
        preserve: false
        type: format
        id: format-0
      - device: disk-sda
        size: 16102981632
        wipe: superblock
        flag: ''
        number: 3
        preserve: false
        type: partition
        id: partition-2
      - name: ubuntu-vg
        devices:
          - partition-2
        preserve: false
        type: lvm_volgroup
        id: lvm_volgroup-0
      - name: ubuntu-lv
        volgroup: lvm_volgroup-0
        size: 16101933056B
        preserve: false
        type: lvm_partition
        id: lvm_partition-0
      - fstype: ext4
        volume: lvm_partition-0
        preserve: false
        type: format
        id: format-1
      - device: format-1
        path: /
        type: mount
        id: mount-1
      - device: format-0
        path: /boot
        type: mount
        id: mount-0
</pre>
And the autoinstall fails and sends me to the interactive installer instead of successfully installing the image onto my chosen (virtual) machine.
I'd happily provide a log if I could just find one myself.
Any advice appreciated.

dgutsche