Comment 17 for bug 1801629

I think I've been successful recovering the installation. Please let me know if this makes sense or if there are any mistakes. People running into this, will be happy to find recovery instructions here.

See also:

# find root partition
sudo fdisk -l

# unencrypt partition
# Note: replace /dev/nvme0n1p3 with your disk
# replace "nvme0n1p3_crypt" with the correct name
# check by running this in chroot:
# $ cat /etc/crypttab | cut -f1 -d " "
# nvme0n1p3_crypt
sudo cryptsetup luksOpen /dev/nvme0n1p3 nvme0n1p3_crypt

# mount root partition
sudo vgscan
sudo vgchange -ay
sudo mount /dev/mapper/xubuntu--vg-root /mnt

# prepare chroot environment
sudo mount /dev/nvme0n1p2 /mnt/boot/
sudo mount -o rbind /dev/ /mnt/dev/
sudo mount -t proc proc /mnt/proc/
sudo mount -t sysfs sys /mnt/sys/

# make dns available in chroot
sudo cp /etc/resolv.conf /mnt/etc/resolv.conf

# enter chroot
sudo chroot /mnt /bin/bash

# re-install missing packages
apt install cryptsetup lvm2

# re-generate (this might be done also by apt in the step before, I'm not sure)
update-initramfs -u -k all

# Leave chroot environment
# Write buffers to disk
sudo sync
# Unmount file systems
sudo umount /mnt/sys
sudo umount /mnt/proc
sudo umount /mnt/boot