unattended-upgrade fails silently in qemu VM

[Christian]

Since upgrading to 1.1ubuntu1.18.04.9 on several servers (VMs) unattended upgrades stopped to run successfully an did not report any errors (via mail).
If I run it manually it errors out as follows:

Traceback (most recent call last):
  File "/usr/bin/unattended-upgrade", line 2057, in <module>
    sys.exit(main(options))
  File "/usr/bin/unattended-upgrade", line 1773, in main
    if conffile_prompt(item.destfile):
  File "/usr/bin/unattended-upgrade", line 988, in conffile_prompt
    with open(prefix + conf_file, 'rb') as fp:
IsADirectoryError: [Errno 21] Is a directory: '/etc/qemu/fsfreeze-hook'

This also happens with Package Version: 1.1ubuntu1.18.04.10

Expected behavior:
Either the the program runs without error or the cron job sends a failure mail.

Here come the logfiles, where you cannot see any errors when on 1.1ubuntu1.18.04.9, but after manually upgrading to 1.1ubuntu1.18.04.10 there are warnings:

2019-05-01 20:28:13,071 INFO Initial whitelisted packages:
2019-05-01 20:28:13,071 INFO Starting unattended upgrades script
2019-05-01 20:28:13,071 INFO Allowed origins are: o=Ubuntu,a=bionic, o=Ubuntu,a=bionic-security, o=UbuntuESM,a=bionic, o=LP-PPA-certbot-certbot,a=bionic, o=LP-PPA-ondrej-nginx-mainline,a=bionic, o=LP-PPA-ondrej-php,a=bionic
2019-05-01 20:39:29,902 INFO Initial blacklisted packages: libc.* linux-.* .*-microcode mysql-.* mariadb-.*
2019-05-01 20:39:29,903 INFO Initial whitelisted packages:
2019-05-01 20:39:29,903 INFO Starting unattended upgrades script
2019-05-01 20:39:29,903 INFO Allowed origins are: o=Ubuntu,a=bionic, o=Ubuntu,a=bionic-security, o=UbuntuESM,a=bionic, o=LP-PPA-certbot-certbot,a=bionic, o=LP-PPA-ondrej-nginx-mainline,a=bionic, o=LP-PPA-ondrej-php,a=bionic
2019-05-01 20:57:47,643 INFO Initial blacklisted packages: libc.* linux-.* .*-microcode mysql-.* mariadb-.*
2019-05-01 20:57:47,644 INFO Initial whitelisted packages:
2019-05-01 20:57:47,644 INFO Starting unattended upgrades script
2019-05-01 20:57:47,644 INFO Allowed origins are: o=Ubuntu,a=bionic, o=Ubuntu,a=bionic-security, o=UbuntuESM,a=bionic, o=LP-PPA-certbot-certbot,a=bionic, o=LP-PPA-ondrej-nginx-mainline,a=bionic, o=LP-PPA-ondrej-php,a=bionic
2019-05-01 21:08:48,278 INFO Initial blacklisted packages: libc.* linux-.* .*-microcode mysql-.* mariadb-.*
2019-05-01 21:08:48,278 INFO Initial whitelisted packages:
2019-05-01 21:08:48,279 INFO Starting unattended upgrades script
2019-05-01 21:08:48,279 INFO Allowed origins are: o=Ubuntu,a=bionic, o=Ubuntu,a=bionic-security, o=UbuntuESM,a=bionic, o=LP-PPA-certbot-certbot,a=bionic, o=LP-PPA-ondrej-nginx-mainline,a=bionic, o=LP-PPA-ondrej-php,a=bionic
2019-05-01 21:08:57,611 WARNING package libxslt1.1 upgradable but fails to be marked for upgrade (E:Error, pkgProblemResolver::Resolve generated breaks, this may be caused by held packages.)
2019-05-01 21:08:59,398 WARNING package ntfs-3g upgradable but fails to be marked for upgrade (E:Error, pkgProblemResolver::Resolve generated breaks, this may be caused by held packages.)