Comment 12 for bug 1654573

After upgrading apt I still got the GUI prompt over and over, and /var/lib/update-notifier/package-data-downloads/partial was found empty.

However, I removed and installed ttf-mscorefonts-installer and while I didn't get any failed to fetch errors, and still got the:
W: Can't drop privileges for downloading as file 'FILE' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)

The fonts installed without errors as per below. Hence, my root cause issue seems to have been I didn't go through a remove/install cycle of ttf-mscorefonts-installer.

sudo apt install ttf-mscorefonts-installer
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following NEW packages will be installed:
  ttf-mscorefonts-installer
0 upgraded, 1 newly installed, 0 to remove and 2 not upgraded.
Need to get 29.5 kB of archives.
After this operation, 134 kB of additional disk space will be used.
Get:1 http://us.archive.ubuntu.com/ubuntu xenial/multiverse amd64 ttf-mscorefonts-installer all 3.4+nmu1ubuntu2 [29.5 kB]
Fetched 29.5 kB in 0s (93.8 kB/s)
Preconfiguring packages ...
Selecting previously unselected package ttf-mscorefonts-installer.
(Reading database ... 281292 files and directories currently installed.)
Preparing to unpack .../ttf-mscorefonts-installer_3.4+nmu1ubuntu2_all.deb ...
mscorefonts-eula license has already been accepted
Unpacking ttf-mscorefonts-installer (3.4+nmu1ubuntu2) ...
Processing triggers for fontconfig (2.11.94-0ubuntu1.1) ...
Processing triggers for update-notifier-common (3.168.3) ...
ttf-mscorefonts-installer: processing...
ttf-mscorefonts-installer: downloading http://downloads.sourceforge.net/corefonts/andale32.exe
Get:1 http://downloads.sourceforge.net/corefonts/andale32.exe [198 kB]
Fetched 198 kB in 10s (18.2 kB/s)
W: Can't drop privileges for downloading as file '/var/lib/update-notifier/package-data-downloads/partial/andale32.exe' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)
ttf-mscorefonts-installer: downloading http://downloads.sourceforge.net/corefonts/arial32.exe
Get:1 http://downloads.sourceforge.net/corefonts/arial32.exe [554 kB]
Fetched 554 kB in 3s (145 kB/s)
W: Can't drop privileges for downloading as file '/var/lib/update-notifier/package-data-downloads/partial/arial32.exe' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)
ttf-mscorefonts-installer: downloading http://downloads.sourceforge.net/corefonts/arialb32.exe
Get:1 http://downloads.sourceforge.net/corefonts/arialb32.exe [168 kB]
Fetched 168 kB in 7s (22.2 kB/s)
W: Can't drop privileges for downloading as file '/var/lib/update-notifier/package-data-downloads/partial/arialb32.exe' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)
ttf-mscorefonts-installer: downloading http://downloads.sourceforge.net/corefonts/comic32.exe
Get:1 http://downloads.sourceforge.net/corefonts/comic32.exe [246 kB]
Fetched 246 kB in 3s (79.0 kB/s)
W: Can't drop privileges for downloading as file '/var/lib/update-notifier/package-data-downloads/partial/comic32.exe' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)
ttf-mscorefonts-installer: downloading http://downloads.sourceforge.net/corefonts/courie32.exe
Get:1 http://downloads.sourceforge.net/corefonts/courie32.exe [646 kB]
Fetched 646 kB in 10s (63.7 kB/s)
W: Can't drop privileges for downloading as file '/var/lib/update-notifier/package-data-downloads/partial/courie32.exe' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)
ttf-mscorefonts-installer: downloading http://downloads.sourceforge.net/corefonts/georgi32.exe
Get:1 http://downloads.sourceforge.net/corefonts/georgi32.exe [392 kB]
Fetched 392 kB in 1s (230 kB/s)
W: Can't drop privileges for downloading as file '/var/lib/update-notifier/package-data-downloads/partial/georgi32.exe' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)
ttf-mscorefonts-installer: downloading http://downloads.sourceforge.net/corefonts/impact32.exe
Get:1 http://downloads.sourceforge.net/corefonts/impact32.exe [173 kB]
Fetched 173 kB in 1s (127 kB/s)
W: Can't drop privileges for downloading as file '/var/lib/update-notifier/package-data-downloads/partial/impact32.exe' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)
ttf-mscorefonts-installer: downloading http://downloads.sourceforge.net/corefonts/times32.exe
Get:1 http://downloads.sourceforge.net/corefonts/times32.exe [662 kB]
Fetched 662 kB in 5s (118 kB/s)
W: Can't drop privileges for downloading as file '/var/lib/update-notifier/package-data-downloads/partial/times32.exe' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)
ttf-mscorefonts-installer: downloading http://downloads.sourceforge.net/corefonts/trebuc32.exe
Get:1 http://downloads.sourceforge.net/corefonts/trebuc32.exe [357 kB]
Fetched 357 kB in 3s (102 kB/s)
W: Can't drop privileges for downloading as file '/var/lib/update-notifier/package-data-downloads/partial/trebuc32.exe' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)
ttf-mscorefonts-installer: downloading http://downloads.sourceforge.net/corefonts/verdan32.exe
Get:1 http://downloads.sourceforge.net/corefonts/verdan32.exe [352 kB]
Fetched 352 kB in 2s (120 kB/s)
W: Can't drop privileges for downloading as file '/var/lib/update-notifier/package-data-downloads/partial/verdan32.exe' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)
ttf-mscorefonts-installer: downloading http://downloads.sourceforge.net/corefonts/webdin32.exe
Get:1 http://downloads.sourceforge.net/corefonts/webdin32.exe [185 kB]
Fetched 185 kB in 2s (92.4 kB/s)
W: Can't drop privileges for downloading as file '/var/lib/update-notifier/package-data-downloads/partial/webdin32.exe' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)

These fonts were provided by Microsoft "in the interest of cross-
platform compatibility". This is no longer the case, but they are
still available from third parties.

You are free to download these fonts and use them for your own use,
but you may not redistribute them in modified form, including changes
to the file name or packaging format.

Extracting cabinet: /var/lib/update-notifier/package-data-downloads/partial/andale32.exe
  extracting fontinst.inf
  extracting andale.inf
  extracting fontinst.exe
  extracting AndaleMo.TTF
  extracting ADVPACK.DLL
  extracting W95INF32.DLL
  extracting W95INF16.DLL

All done, no errors.
Extracting cabinet: /var/lib/update-notifier/package-data-downloads/partial/arial32.exe
  extracting FONTINST.EXE
  extracting fontinst.inf
  extracting Ariali.TTF
  extracting Arialbd.TTF
  extracting Arialbi.TTF
  extracting Arial.TTF

All done, no errors.
Extracting cabinet: /var/lib/update-notifier/package-data-downloads/partial/arialb32.exe
  extracting fontinst.exe
  extracting fontinst.inf
  extracting AriBlk.TTF

All done, no errors.
Extracting cabinet: /var/lib/update-notifier/package-data-downloads/partial/comic32.exe
  extracting fontinst.inf
  extracting Comicbd.TTF
  extracting Comic.TTF
  extracting fontinst.exe

All done, no errors.
Extracting cabinet: /var/lib/update-notifier/package-data-downloads/partial/courie32.exe
  extracting cour.ttf
  extracting courbd.ttf
  extracting courbi.ttf
  extracting fontinst.inf
  extracting couri.ttf
  extracting fontinst.exe

All done, no errors.
Extracting cabinet: /var/lib/update-notifier/package-data-downloads/partial/georgi32.exe
  extracting fontinst.inf
  extracting Georgiaz.TTF
  extracting Georgiab.TTF
  extracting Georgiai.TTF
  extracting Georgia.TTF
  extracting fontinst.exe

All done, no errors.
Extracting cabinet: /var/lib/update-notifier/package-data-downloads/partial/impact32.exe
  extracting fontinst.exe
  extracting Impact.TTF
  extracting fontinst.inf

All done, no errors.
Extracting cabinet: /var/lib/update-notifier/package-data-downloads/partial/times32.exe
  extracting fontinst.inf
  extracting Times.TTF
  extracting Timesbd.TTF
  extracting Timesbi.TTF
  extracting Timesi.TTF
  extracting FONTINST.EXE

All done, no errors.
Extracting cabinet: /var/lib/update-notifier/package-data-downloads/partial/trebuc32.exe
  extracting FONTINST.EXE
  extracting trebuc.ttf
  extracting Trebucbd.ttf
  extracting trebucbi.ttf
  extracting trebucit.ttf
  extracting fontinst.inf

All done, no errors.
Extracting cabinet: /var/lib/update-notifier/package-data-downloads/partial/verdan32.exe
  extracting fontinst.exe
  extracting fontinst.inf
  extracting Verdanab.TTF
  extracting Verdanai.TTF
  extracting Verdanaz.TTF
  extracting Verdana.TTF

All done, no errors.
Extracting cabinet: /var/lib/update-notifier/package-data-downloads/partial/webdin32.exe
  extracting fontinst.exe
  extracting Webdings.TTF
  extracting fontinst.inf
  extracting Licen.TXT

All done, no errors.
All fonts downloaded and installed.
Setting up ttf-mscorefonts-installer (3.4+nmu1ubuntu2) ...