Comment 15 for bug 1543280

For me this problem was solved by, as #9 suggested:
# add unprivileged user for the apt methods
adduser --force-badname --system --home /nonexistent \
--no-create-home --quiet _apt || true

I don't understand what this command should do and it does not work:
# Fixup any mistake in the home directory of the _apt user
if dpkg --compare-versions "$2" lt-nl 1.1~exp10~; then
usermod --home /nonexistent _apt

I also had to delete all files in /var/lib/update-notifier/package-data-downloads/partial/

My system: fresh install of Kubuntu Xenial