Comment 84 for bug 1607535

apt-cache policy ttf-mscorefonts-installer
ttf-mscorefonts-installer:
  Installed: 3.4+nmu1ubuntu2
  Candidate: 3.4+nmu1ubuntu2
  Version table:
 *** 3.4+nmu1ubuntu2 500
        500 http://it.archive.ubuntu.com/ubuntu xenial/multiverse amd64 Packages
        500 http://it.archive.ubuntu.com/ubuntu xenial/multiverse i386 Packages
        100 /var/lib/dpkg/status

There were actually three files:

ls -l /var/lib/update-notifier/package-data-downloads/partial/
total 804K
-rw-r--r-- 1 root root 969 2016-11-29 07:35:04 andale32.exe.FAILED
-rw-r--r-- 1 root root 165K 2002-08-15 16:34:10 arialb32.exe
-rw-r--r-- 1 root root 632K 2002-08-15 16:34:27 courie32.exe

I removed those files and I got:

...
All done, no errors.
All fonts downloaded and installed.
flashplugin-installer: processing...
flashplugin-installer: downloading http://archive.canonical.com/pool/partner/a/adobe-flashplugin/adobe-flashplugin_20170314.1.orig.tar.gz
Scaricamento di:1 http://archive.canonical.com/pool/partner/a/adobe-flashplugin/adobe-flashplugin_20170314.1.orig.tar.gz [30,4 MB]
Recuperati 30,4 MB in 23s (1.283 kB/s)
W: Can't drop privileges for downloading as file '/var/lib/update-notifier/package-data-downloads/partial/adobe-flashplugin_20170314.1.orig.tar.gz' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permesso negato)
Installing from local file /var/lib/update-notifier/package-data-downloads/partial/adobe-flashplugin_20170314.1.orig.tar.gz
Flash Plugin installed.

So the application is still buggy: it can't be that I need to manually remove failed downloads in order to have a successful execution.