Ubuntu installer failed while trying "fresh" install over existing instalation on btrfs

I have a previous installation of Ubuntu 16.04.
My disk has 2 GPT partitions:
/dev/sda1 for /boot/efi
/dev/sda2 btrfs with 2 subvolumes: @ mounted as /, @home mounted as /home.

I tried a fresh install of Ubuntu 20.04.1. I selected manual partitioning and /dev/sda1 was recognized as Boot/efi. I had to select /dev/sda2 and specify "mount as" / (there is no better alternative). I proceeded and verified on a terminal that the installer detected the subvolumes just fine:

    ubuntu@ubuntu:~$ mount | grep sda
    /dev/sda2 on /target type btrfs (rw,relatime,ssd,space_cache,subvolid=257,subvol=/@)
    /dev/sda1 on /target/boot/efi type vfat (rw,relatime,fmask=0077,dmask=0077,codepage=437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro)
    /dev/sda2 on /target/home type btrfs (rw,relatime,ssd,space_cache,subvolid=258,subvol=/@home)

The instalation went on to copy files, all seemed fine. When the step "Creating a user" was reached, the installer crashed with an unspecified error.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: ubiquity 20.04.15.2
ProcVersionSignature: Ubuntu 5.4.0-42.46-generic 5.4.44
Uname: Linux 5.4.0-42-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu27.4
Architecture: amd64
CasperMD5CheckResult: pass
CasperVersion: 1.445.1
Date: Mon Sep 21 15:51:47 2020
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed maybe-ubiquity quiet splash ---
LiveMediaBuild: Ubuntu 20.04.1 LTS "Focal Fossa" - Release amd64 (20200731)
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)