It fails when I try to use a local focal mirror or if I set a proxy. Here is the install log snippet:
2020-12-01 13:47:16,916 DEBUG root:39 start: subiquity/InstallProgress/install/curtin_install/cmd-install/stage-curthooks: configuring installed system
2020-12-01 13:47:16,918 DEBUG root:39 start: subiquity/InstallProgress/install/curtin_install/cmd-install/stage-curthooks/000-configure-run: running '/snap/bin/subiquity.subiquity-configure-run'
2020-12-01 13:47:16,919 DEBUG root:39 finish: subiquity/InstallProgress/install/curtin_install/cmd-install/stage-curthooks/000-configure-run: SUCCESS: Status.SUCCESS
2020-12-01 13:47:16,925 DEBUG root:39 start: subiquity/InstallProgress/install/curtin_install/cmd-install/stage-curthooks/001-configure-apt: running '/snap/bin/subiquity.subiquity-configure-apt /snap/subiquity/1966/usr/bin/python3 true'
2020-12-01 13:47:17,503 DEBUG root:39 start: subiquity/InstallProgress/install/curtin_install/cmd-install/stage-curthooks/001-configure-apt/cmd-apt-config: curtin command apt-config
2020-12-01 13:47:17,508 DEBUG root:39 finish: subiquity/InstallProgress/install/curtin_install/cmd-install/stage-curthooks/001-configure-apt/cmd-apt-config: SUCCESS: Status.SUCCESS
2020-12-01 13:47:18,405 DEBUG root:39 start: subiquity/InstallProgress/install/curtin_install/cmd-install/stage-curthooks/001-configure-apt/cmd-in-target: curtin command in-target
2020-12-01 13:47:32,907 DEBUG root:39 finish: subiquity/InstallProgress/install/curtin_install/cmd-install/stage-curthooks/001-configure-apt/cmd-in-target: SUCCESS: Status.FAIL
2020-12-01 13:47:32,909 DEBUG root:39 finish: subiquity/InstallProgress/install/curtin_install/cmd-install/stage-curthooks/001-configure-apt: SUCCESS: Status.FAIL
2020-12-01 13:47:32,911 DEBUG root:39 finish: subiquity/InstallProgress/install/curtin_install/cmd-install/stage-curthooks: SUCCESS: Status.FAIL
2020-12-01 13:47:33,100 DEBUG subiquitycore.utils:83 arun_command ['systemd-cat', '--level-prefix=false', '--identifier=curtin_log.6605', '/snap/subiquity/1966/usr/bin/python3', '-m', 'curtin', '--showtrace', '-c', '/var/log/installer/subiquity-curtin-install.conf', 'install'] exited with code 3
2020-12-01 13:47:33,100 DEBUG subiquity.lockfile:44 unlocking exclusive /run/subiquity/installing
2020-12-01 13:47:33,101 ERROR root:39 finish: subiquity/InstallProgress/install/curtin_install: FAIL: Command '['systemd-cat', '--level-prefix=false', '--identifier=curtin_log.6605', '/snap/subiquity/1966/usr/bin/python3', '-m', 'curtin', '--showtrace', '-c', '/var/log/installer/subiquity-curtin-install.conf', 'install']' returned non-zero exit status 3.
2020-12-01 13:47:33,103 ERROR subiquitycore.controller.installprogress:145 curtin_error
Traceback (most recent call last):
File "/snap/subiquity/1966/lib/python3.6/site-packages/subiquity/controllers/installprogress.py", line 304, in install
await self.curtin_install(context=context)
File "/snap/subiquity/1966/lib/python3.6/site-packages/subiquitycore/context.py", line 142, in decorated_async
return await meth(self, **kw)
File "/snap/subiquity/1966/lib/python3.6/site-packages/subiquity/controllers/installprogress.py", line 281, in curtin_install
self.logged_command(curtin_cmd), check=True)
File "/snap/subiquity/1966/lib/python3.6/site-packages/subiquitycore/utils.py", line 85, in arun_command
raise subprocess.CalledProcessError(proc.returncode, cmd)
subprocess.CalledProcessError: Command '['systemd-cat', '--level-prefix=false', '--identifier=curtin_log.6605', '/snap/subiquity/1966/usr/bin/python3', '-m', 'curtin', '--showtrace', '-c', '/var/log/installer/subiquity-curtin-install.conf', 'install']' returned non-zero exit status 3.
2020-12-01 13:47:33,104 DEBUG subiquity.core:470 generating crash report
The issue is caused when multiple network adapters are used. The issue goes away when only one NIC is used.