Kubuntu 18.04 fails to install from USB

[Steve]

Attempting to install Kubuntu 18.04 fresh from a USB (after having a failed in-place upgrade from 17.10 that destroyed my graphical environment). Installer throws an error message after setting username, password, etc. -- " ubi-usersetup failed with exit code 141". Clicked "Retry" and was able to continue normally. After setting partitions, installation began; it hanged at 93%, "Restoring previously installed applications" before throwing an error message indicating that not all applications could be restored, and that some would need to be manually reinstalled. Immediately thereafter the installer itself crashed.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubiquity 18.04.14
ProcVersionSignature: Ubuntu 4.15.0-20.21-generic 4.15.17
Uname: Linux 4.15.0-20-generic x86_64
ApportVersion: 2.20.9-0ubuntu7
Architecture: amd64
CasperVersion: 1.394
CurrentDesktop: KDE
Date: Sat May 12 13:05:36 2018
InstallCmdLine: BOOT_IMAGE=(loop)/casper/vmlinuz root=UUID=8A52-2251 maybe-ubiquity debian-installer/language=en keyboard-configuration/layoutcode=us iso-scan/filename=/kubuntu-18.04-desktop-amd64.iso boot=casper file=/cdrom/preseed/kubuntu.seed noprompt quiet splash --
LiveMediaBuild: Kubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Brian Murray (brian-murray)]

This actually seems to be an issue with apt-clone:

May 12 20:00:43 kubuntu /plugininstall.py: Exception during installation:
May 12 20:00:43 kubuntu /plugininstall.py: Traceback (most recent call last):
May 12 20:00:43 kubuntu /plugininstall.py: File "/usr/share/ubiquity/plugininstall.py", line 246, in run
May 12 20:00:43 kubuntu /plugininstall.py: self.apt_clone_restore()
May 12 20:00:43 kubuntu /plugininstall.py: File "/usr/share/ubiquity/plugininstall.py", line 1496, in apt_clone_restore
May 12 20:00:43 kubuntu /plugininstall.py: restore_cmd, preexec_fn=install_misc.debconf_disconnect)
May 12 20:00:43 kubuntu /plugininstall.py: File "/usr/lib/python3.6/subprocess.py", line 291, in check_call
May 12 20:00:43 kubuntu /plugininstall.py: raise CalledProcessError(retcode, cmd)
May 12 20:00:43 kubuntu /plugininstall.py: subprocess.CalledProcessError: Command '['apt-clone', 'restore-new-distro', '/target/ubiquity-apt-clone/apt-clone-state-kubuntu.tar.gz', 'bionic', '--destination', '/target']' returned non-zero exit status 1.
May 12 20:00:43 kubuntu /plugininstall.py:
May 12 20:00:43 kubuntu /plugininstall.py: During handling of the above exception, another exception occurred:
May 12 20:00:43 kubuntu /plugininstall.py:
May 12 20:00:43 kubuntu /plugininstall.py: Traceback (most recent call last):
May 12 20:00:43 kubuntu /plugininstall.py: File "/usr/share/ubiquity/plugininstall.py", line 1739, in <module>
May 12 20:00:43 kubuntu /plugininstall.py: install.run()
May 12 20:00:43 kubuntu /plugininstall.py: File "/usr/share/ubiquity/plugininstall.py", line 61, in wrapper
May 12 20:00:43 kubuntu /plugininstall.py: func(self)
May 12 20:00:43 kubuntu /plugininstall.py: File "/usr/share/ubiquity/plugininstall.py", line 253, in run
May 12 20:00:43 kubuntu /plugininstall.py: self.db.input('critical', 'ubiquity/install/broken_apt_clone')
May 12 20:00:43 kubuntu /plugininstall.py: File "/usr/lib/python3/dist-packages/debconf.py", line 83, in <lambda>
May 12 20:00:43 kubuntu /plugininstall.py: lambda *args, **kw: self.command(command, *args, **kw))
May 12 20:00:43 kubuntu /plugininstall.py: File "/usr/lib/python3/dist-packages/debconf.py", line 104, in command
May 12 20:00:43 kubuntu /plugininstall.py: status = int(status)
May 12 20:00:43 kubuntu /plugininstall.py: ValueError: invalid literal for int() with base 10: ''