Tried it, worked (unexpectedly). Running without the debug option produces the same dialog (Failed to download packages...)

Log:

$ update-manager --debug
DEBUG:AptClient.DebconfProxy:debconf socket: /tmp/aptdaemon-zro74xyo/debconf.socket
DEBUG:root:debconf.start()
DEBUG:root:debconf.stop()
DEBUG:root:App candidate for system-config-printer-gnome: /usr/share/app-install/desktop/system-config-printer-gnome:system-config-printer.desktop
DEBUG:root:App candidate for unity-webapps-qml: /usr/share/app-install/desktop/unity-webapps-qml:unitywebappsqmllauncher.desktop
DEBUG:root:WARNING: can not get name for '<gtk.TreeViewAccessible object at 0x7fee383a3ca8 (GtkTreeViewAccessible at 0x2fdaa70)>'
DEBUG:root:WARNING: can not get name for '<gtk.LabelAccessible object at 0x7fee38347120 (GtkLabelAccessible at 0x4174750)>'
DEBUG:root:WARNING: can not get name for '<TreeSelection object at 0x7fee38347990 (GtkTreeSelection at 0x2c40830)>'
DEBUG:root:WARNING: can not get name for '<gtk.TextViewAccessible object at 0x7fee383addc8 (GtkTextViewAccessible at 0x2fdac80)>'
DEBUG:root:cache aufs_rw_dir: /tmp/
DEBUG:root:Free space on /: 481305055232
DEBUG:root:Dir /usr mounted on /
DEBUG:root:Dir /var mounted on /
DEBUG:root:Dir /boot mounted on /
DEBUG:root:Dir /var/cache/apt/archives mounted on /
DEBUG:root:Dir /tmp mounted on /
DEBUG:root:Dir /home mounted on /
DEBUG:root:Dir /tmp mounted on /
DEBUG:root:fs_free contains: '{'/tmp': <DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7fee463942b0>, '/usr': <DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7fee463942b0>, '/boot': <DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7fee463942b0>, '/var/cache/apt/archives': <DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7fee463942b0>, '/var': <DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7fee463942b0>, '/home': <DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7fee463942b0>, '/': <DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7fee463942b0>}'
DEBUG:root:dir '/var/cache/apt/archives' needs '7139400' of '<DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7fee463942b0>' (481305055232.000000)
DEBUG:root:dir '/usr' needs '-9216' of '<DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7fee463942b0>' (481297915832.000000)
DEBUG:root:dir '/usr' needs '52428800' of '<DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7fee463942b0>' (481297925048.000000)
DEBUG:root:dir '/boot' needs '0' of '<DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7fee463942b0>' (481245496248.000000)
DEBUG:root:dir '/tmp' needs '5242880' of '<DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7fee463942b0>' (481245496248.000000)
DEBUG:root:dir '/' needs '10485760' of '<DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7fee463942b0>' (481240253368.000000)
DEBUG:root:dir '/tmp' needs '0.0' of '<DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7fee463942b0>' (481229767608.000000)
DEBUG:root:dir '/usr' needs '0.0' of '<DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7fee463942b0>' (481229767608.000000)
DEBUG:AptClient.DebconfProxy:debconf socket: /tmp/aptdaemon-hcvh4suj/debconf.socket
DEBUG:root:debconf.start()
DEBUG:root:_copy_conn
DEBUG:AptClient.DebconfProxy:From socket: b'TITLE Configuring samba-common\n'
debconf: DbDriver "passwords" warning: could not open /var/cache/debconf/passwords.dat: Permission denied
DEBUG:root:_copy_stdout
DEBUG:AptClient.DebconfProxy:From debconf: b'0\n'
DEBUG:root:_copy_conn
DEBUG:AptClient.DebconfProxy:From socket: b'DATA samba-common/title type title\n'
DEBUG:root:_copy_stdout
DEBUG:AptClient.DebconfProxy:From debconf: b'10 Template type already set\n'
DEBUG:root:_copy_conn
DEBUG:AptClient.DebconfProxy:From socket: b'DATA samba-common/title description Samba server and utilities\n'
DEBUG:root:_copy_stdout
DEBUG:AptClient.DebconfProxy:From debconf: b'0\n'
DEBUG:root:_copy_conn
DEBUG:AptClient.DebconfProxy:From socket: b'SETTITLE samba-common/title\n'
DEBUG:root:_copy_stdout
DEBUG:AptClient.DebconfProxy:From debconf: b'0\n'
DEBUG:root:_copy_conn
DEBUG:AptClient.DebconfProxy:From socket: b'TITLE Configuring flashplugin-installer\n'
DEBUG:root:_copy_stdout
DEBUG:AptClient.DebconfProxy:From debconf: b'0\n'
DEBUG:root:_copy_conn
DEBUG:AptClient.DebconfProxy:Stop reading from conn
DEBUG:root:_copy_stdout
DEBUG:AptClient.DebconfProxy:Stop reading from stdout
DEBUG:root:_copy_conn
DEBUG:AptClient.DebconfProxy:From socket: b'TITLE Configuring samba-common\n'
debconf: DbDriver "passwords" warning: could not open /var/cache/debconf/passwords.dat: Permission denied
DEBUG:root:_copy_stdout
DEBUG:AptClient.DebconfProxy:From debconf: b'0\n'
DEBUG:root:_copy_conn
DEBUG:AptClient.DebconfProxy:From socket: b'DATA samba-common/title type title\n'
DEBUG:root:_copy_stdout
DEBUG:AptClient.DebconfProxy:From debconf: b'10 Template type already set\n'
DEBUG:root:_copy_conn
DEBUG:AptClient.DebconfProxy:From socket: b'DATA samba-common/title description Samba server and utilities\n'
DEBUG:root:_copy_stdout
DEBUG:AptClient.DebconfProxy:From debconf: b'0\n'
DEBUG:root:_copy_conn
DEBUG:AptClient.DebconfProxy:From socket: b'SETTITLE samba-common/title\n'
DEBUG:root:_copy_stdout
DEBUG:AptClient.DebconfProxy:From debconf: b'0\n'
DEBUG:root:_copy_conn
DEBUG:AptClient.DebconfProxy:From socket: b'TITLE Configuring flashplugin-installer\n'
DEBUG:root:_copy_stdout
DEBUG:AptClient.DebconfProxy:From debconf: b'0\n'
DEBUG:root:_copy_conn
DEBUG:AptClient.DebconfProxy:Stop reading from conn
DEBUG:root:_copy_stdout
DEBUG:AptClient.DebconfProxy:Stop reading from stdout
DEBUG:root:_copy_conn
DEBUG:AptClient.DebconfProxy:From socket: b'TITLE Configuring samba-common\n'
debconf: DbDriver "passwords" warning: could not open /var/cache/debconf/passwords.dat: Permission denied
DEBUG:root:_copy_stdout
DEBUG:AptClient.DebconfProxy:From debconf: b'0\n'
DEBUG:root:_copy_conn
DEBUG:AptClient.DebconfProxy:From socket: b'DATA samba-common/title type title\n'
DEBUG:root:_copy_stdout
DEBUG:AptClient.DebconfProxy:From debconf: b'10 Template type already set\n'
DEBUG:root:_copy_conn
DEBUG:AptClient.DebconfProxy:From socket: b'DATA samba-common/title description Samba server and utilities\n'
DEBUG:root:_copy_stdout
DEBUG:AptClient.DebconfProxy:From debconf: b'0\n'
DEBUG:root:_copy_conn
DEBUG:AptClient.DebconfProxy:From socket: b'SETTITLE samba-common/title\n'
DEBUG:root:_copy_stdout
DEBUG:AptClient.DebconfProxy:From debconf: b'0\n'
DEBUG:root:_copy_conn
DEBUG:AptClient.DebconfProxy:From socket: b'TITLE Configuring flashplugin-installer\n'
DEBUG:root:_copy_stdout
DEBUG:AptClient.DebconfProxy:From debconf: b'0\n'
DEBUG:root:_copy_conn
DEBUG:AptClient.DebconfProxy:Stop reading from conn
DEBUG:root:_copy_stdout
DEBUG:AptClient.DebconfProxy:Stop reading from stdout
DEBUG:root:_copy_conn
DEBUG:AptClient.DebconfProxy:From socket: b'CAPB\n'
debconf: DbDriver "passwords" warning: could not open /var/cache/debconf/passwords.dat: Permission denied
DEBUG:root:_copy_stdout
DEBUG:AptClient.DebconfProxy:From debconf: b'0 multiselect escape backup\n'
DEBUG:root:_copy_conn
DEBUG:AptClient.DebconfProxy:Stop reading from conn
DEBUG:root:_copy_stdout
DEBUG:AptClient.DebconfProxy:Stop reading from stdout
DEBUG:root:_copy_conn
DEBUG:AptClient.DebconfProxy:From socket: b'TITLE Configuring samba-common\n'
debconf: DbDriver "passwords" warning: could not open /var/cache/debconf/passwords.dat: Permission denied
DEBUG:root:_copy_stdout
DEBUG:AptClient.DebconfProxy:From debconf: b'0\n'
DEBUG:root:_copy_conn
DEBUG:AptClient.DebconfProxy:From socket: b'DATA samba-common/title type title\n'
DEBUG:root:_copy_stdout
DEBUG:AptClient.DebconfProxy:From debconf: b'10 Template type already set\n'
DEBUG:root:_copy_conn
DEBUG:AptClient.DebconfProxy:From socket: b'DATA samba-common/title description Samba server and utilities\n'
DEBUG:root:_copy_stdout
DEBUG:AptClient.DebconfProxy:From debconf: b'0\n'
DEBUG:root:_copy_conn
DEBUG:AptClient.DebconfProxy:From socket: b'SETTITLE samba-common/title\n'
DEBUG:root:_copy_stdout
DEBUG:AptClient.DebconfProxy:From debconf: b'0\n'
DEBUG:root:_copy_conn
DEBUG:AptClient.DebconfProxy:Stop reading from conn
DEBUG:root:_copy_stdout
DEBUG:AptClient.DebconfProxy:Stop reading from stdout
DEBUG:root:_copy_conn
DEBUG:AptClient.DebconfProxy:From socket: b'TITLE Configuring flashplugin-installer\n'
debconf: DbDriver "passwords" warning: could not open /var/cache/debconf/passwords.dat: Permission denied
DEBUG:root:_copy_stdout
DEBUG:AptClient.DebconfProxy:From debconf: b'0\n'
DEBUG:root:_copy_conn
DEBUG:AptClient.DebconfProxy:Stop reading from conn
DEBUG:root:_copy_stdout
DEBUG:AptClient.DebconfProxy:Stop reading from stdout
DEBUG:root:debconf.stop()


After a while, the flash plugin installer started and downloaded extra data

flashplugin-installer: downloading http://archive.canonical.com/pool/partner/a/adobe-flashplugin/adobe-flashplugin_11.2.202.438.orig.tar.gz
Get:1 http://archive.canonical.com/pool/partner/a/adobe-flashplugin/adobe-flashplugin_11.2.202.438.orig.tar.gz [14,1 MB]
4% [1 /var/lib/update-notifier/package-data-downloads/partial/adobe-flashplugin

(download is awfully slow - 1% / minute)