failed during installation with boot partition encrypted

[Ilídio Amaral]

Installing xubuntu on a multi-disk system (ssd +hdd) with full encryption.
Partition scheme:
  ssd-sda:
    sda1: efi partition plain
    sda2: boot partition encrypted luks
    sda3: encrypted luks2
      lvm volume: root partition

  hdd-sdb:
    sdb1: encypted partition luks2
      lvm volume:
        swap
        var
        home
        labs

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubiquity 18.04.14.6
ProcVersionSignature: Ubuntu 4.15.0-29.31-generic 4.15.18
Uname: Linux 4.15.0-29-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.2
Architecture: amd64
CasperVersion: 1.394
Date: Sun Sep 30 18:44:57 2018
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/xubuntu.seed boot=casper quiet splash ---
LiveMediaBuild: Xubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)