lubuntu groovy calamares aborts 'Failed to unpack image "/cdrom/casper/filesystem.squashfs' error code 10 |11

Lubuntu groovy QA-test install on
sony vaio svp112a1cw (i5-9400u, 4gb, intel haswell-ULT)

QA testcase: Custom partitioning with BTRFS

Boot, start installer via icon on screen
I created two partitions
- FAT32 for /boot/efi
- BTRFS for /
This resulted in (note: I didn't file this report, expected it to occur again on next box)

"Installation failed
Failed to unpack image "/cdrom/casper/filesystem.squashfs"
sync failed with error code 10."

I did the install on another box without issue.

Then I returned to box listed to try again, given BTRFS was flawless (on other box) I attempted this time using REISERfs (spel?) in case I selected the wrong filesystem.. This failed again with error message in this bug report

"Installation failed
Failed to unpack image "/cdrom/casper/filesystem.squashfs"
sync failed with error code 11."

Identical message except error code 10 on first is now 11. It was this error code 11 that this bug is filed against.

ProblemType: Bug
DistroRelease: Ubuntu 20.10
Package: calamares 3.2.24-0ubuntu2
ProcVersionSignature: Ubuntu 5.8.0-18.19-generic 5.8.4
Uname: Linux 5.8.0-18-generic x86_64
.etc.calamares.modules.finished.conf:
 ---
 restartNowMode: user-checked
 restartNowCommand: "systemctl -i reboot"
.etc.calamares.modules.partition.conf:
 efiSystemPartition: "/boot/efi"
 enableLuksAutomatedPartitioning: true
 userSwapChoices: none
 drawNestedPartitions: true
 defaultFileSystemType: "ext4"
.etc.calamares.modules.shellprocess_logs.conf:
 ---
 dontChroot: true
 timeout: 30
 script:
     - calamares-logs-helper @@ROOT@@
.etc.calamares.modules.unpackfs.conf:
 ---
 unpack:
     - source: "/cdrom/casper/filesystem.squashfs"
         sourcefs: "squashfs"
         destination: ""
ApportVersion: 2.20.11-0ubuntu45
Architecture: amd64
CasperMD5CheckResult: pass
CasperVersion: 1.452
CurrentDesktop: LXQt
Date: Sat Sep 5 05:16:32 2020
LiveMediaBuild: Lubuntu 20.10 "Groovy Gorilla" - Alpha amd64 (20200904)
RelatedPackageVersions:
 calamares-settings-ubuntu-common 1:20.10.8
 calamares-settings-lubuntu 1:20.10.8
 xfsprogs 5.6.0-1ubuntu4
 btrfs-progs 5.7-1
SourcePackage: calamares
UpgradeStatus: No upgrade log present (probably fresh install)