Output of update-manager --debug, per request:

$ update-manager --debug

(update-manager:14909): dconf-WARNING **: failed to commit changes to dconf: GDBus.Error:org.gtk.GDBus.UnmappedGError.Quark._g_2dfile_2derror_2dquark.Code12: Failed to write file '/home/vorlon/.config/dconf/user.YF08RY': write() failed: No space left on device
DEBUG:AptClient.DebconfProxy:debconf socket: /tmp/aptdaemon-0_qlhknq/debconf.socket
DEBUG:root:debconf.start()
DEBUG:root:debconf.stop()
INFO:root:holding back phased update libsnapd-glib1 (0 < 53)
INFO:root:holding back phased update snapd-login-service (0 < 53)
DEBUG:root:App candidate for mythtv-frontend: /usr/share/app-install/desktop/mythtv-frontend:mythtv.desktop
DEBUG:root:Useless call to _add_deps. Package was None or didn't have a candidate.
DEBUG:root:Useless call to _add_deps. Package was None or didn't have a candidate.
DEBUG:root:Useless call to _add_deps. Package was None or didn't have a candidate.
DEBUG:root:Useless call to _add_deps. Package was None or didn't have a candidate.
DEBUG:root:Useless call to _add_deps. Package was None or didn't have a candidate.
DEBUG:root:Useless call to _add_deps. Package was None or didn't have a candidate.
DEBUG:root:Useless call to _add_deps. Package was None or didn't have a candidate.
DEBUG:root:Useless call to _add_deps. Package was None or didn't have a candidate.
DEBUG:root:Useless call to _add_deps. Package was None or didn't have a candidate.
DEBUG:root:Useless call to _add_deps. Package was None or didn't have a candidate.
DEBUG:root:Useless call to _add_deps. Package was None or didn't have a candidate.
DEBUG:root:Useless call to _add_deps. Package was None or didn't have a candidate.
DEBUG:root:Useless call to _add_deps. Package was None or didn't have a candidate.
DEBUG:root:Useless call to _add_deps. Package was None or didn't have a candidate.
DEBUG:root:Useless call to _add_deps. Package was None or didn't have a candidate.
DEBUG:root:Useless call to _add_deps. Package was None or didn't have a candidate.
DEBUG:root:Useless call to _add_deps. Package was None or didn't have a candidate.
DEBUG:root:Useless call to _add_deps. Package was None or didn't have a candidate.
DEBUG:root:WARNING: can not get name for '<Gtk.TreeViewAccessible object at 0x7f969520b828 (GtkTreeViewAccessible at 0x55bc8851f300)>'
DEBUG:root:WARNING: can not get name for '<Gtk.LabelAccessible object at 0x7f969520f9d8 (GtkLabelAccessible at 0x55bc88c0e6c0)>'
DEBUG:root:WARNING: can not get name for '<Gtk.TreeSelection object at 0x7f969520fe58 (GtkTreeSelection at 0x55bc88dce080)>'
DEBUG:root:WARNING: can not get name for '<Gtk.TextViewAccessible object at 0x7f969519c240 (GtkTextViewAccessible at 0x55bc88bd4730)>'

(update-manager:14909): Gtk-WARNING **: Could not get code point of keysym \

(update-manager:14909): Gtk-WARNING **: Could not get code point of keysym )

(update-manager:14909): Gtk-WARNING **: compose file /home/vorlon/.XCompose does not include any keys besides keys in en-us compose file

(update-manager:14909): Gtk-WARNING **: Could not get code point of keysym \

(update-manager:14909): Gtk-WARNING **: Could not get code point of keysym )

(update-manager:14909): Gtk-WARNING **: compose file /home/vorlon/.XCompose does not include any keys besides keys in en-us compose file
DEBUG:root:cache aufs_rw_dir: /tmp/
DEBUG:root:Free space on /: 3645808640
DEBUG:root:Free space on /usr: 1834479616
DEBUG:root:Dir /var mounted on /
DEBUG:root:Free space on /boot: 122780672
DEBUG:root:Dir /var/cache/apt/archives mounted on /
DEBUG:root:Dir /tmp mounted on /
DEBUG:root:Free space on /home: 0
DEBUG:root:Dir /tmp mounted on /
DEBUG:root:fs_free contains: '{'/home': <DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7f96951b7da0>, '/boot': <DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7f96951b7cf8>, '/var/cache/apt/archives': <DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7f96951b7c18>, '/usr': <DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7f96951b7b70>, '/var': <DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7f96951b7c18>, '/': <DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7f96951b7c18>, '/tmp': <DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7f96951b7c18>}'
DEBUG:root:linux-image-4.8.0-30-generic (new-install) added with 65969653 to boot space
DEBUG:root:dir '/var/cache/apt/archives' needs '83022492' of '<DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7f96951b7c18>' (3645808640.000000)
DEBUG:root:dir '/usr' needs '314591232' of '<DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7f96951b7b70>' (1834479616.000000)
DEBUG:root:dir '/usr' needs '52428800' of '<DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7f96951b7b70>' (1519888384.000000)
DEBUG:root:dir '/boot' needs '131939306' of '<DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7f96951b7cf8>' (122780672.000000)
DEBUG:root:dir '/tmp' needs '5242880' of '<DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7f96951b7c18>' (3562786148.000000)
DEBUG:root:dir '/' needs '10485760' of '<DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7f96951b7c18>' (3557543268.000000)
DEBUG:root:dir '/tmp' needs '0.0' of '<DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7f96951b7c18>' (3547057508.000000)
DEBUG:root:dir '/usr' needs '0.0' of '<DistUpgrade.DistUpgradeCache.MyCache.checkFreeSpace.<locals>.FreeSpace object at 0x7f96951b7b70>' (1467459584.000000)
ERROR:root:Not enough free space: ['/boot']

(update-manager:14909): dconf-WARNING **: failed to commit changes to dconf: GDBus.Error:org.gtk.GDBus.UnmappedGError.Quark._g_2dfile_2derror_2dquark.Code12: Failed to write file '/home/vorlon/.config/dconf/user.JXT7RY': write() failed: No space left on device

(update-manager:14909): dconf-WARNING **: failed to commit changes to dconf: GDBus.Error:org.gtk.GDBus.UnmappedGError.Quark._g_2dfile_2derror_2dquark.Code12: Failed to write file '/home/vorlon/.config/dconf/user.2TS7RY': write() failed: No space left on device