user _apt can't write to directory "partial"

[Thomas Mayer]

Permissions for directory "partial" are:

 ls -lh /var/lib/update-notifier/package-data-downloads/
insgesamt 0
drwxr-xr-x 1 root root 0 Jan 7 04:21 partial
-rw-r--r-- 1 root root 0 Jan 7 04:21 ttf-mscorefonts-installer.failed

A user with name "_apt" can't write to directory "partial".

That happened to me when trying to let msttcorefonts-installer download its fonts:

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: Keine Berechtigung)

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: update-notifier 3.168.3
ProcVersionSignature: Ubuntu 4.4.0-57.78-generic 4.4.35
Uname: Linux 4.4.0-57-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.4
Architecture: amd64
Date: Sat Jan 7 04:38:14 2017
InstallationDate: Installed on 2014-11-29 (770 days ago)
InstallationMedia: Ubuntu 14.04.1 LTS "Trusty Tahr" - Release amd64 (20140722.2)
ProcEnviron:
 LANGUAGE=de_DE
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: update-notifier
UpgradeStatus: Upgraded to xenial on 2016-06-15 (205 days ago)
upstart.update-notifier-release.log: Sa 7. Jan 03:43:11 CET 2017 new release available

[Thomas Mayer (thomas303)]

Related: https://ubuntuforums.org/showthread.php?t=2323229

This still is not fixed.