Comment 9 for bug 1543280

Please check:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=808802

# add unprivileged user for the apt methods

adduser --force-badname --system --home /nonexistent \
--no-create-home --quiet _apt || true

# 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

This is due, as the message implies the inexistence of the user _apt (which somehow is now required and not preexistent on the user list).

Previously all those folders where owned by root:root, now they are ubuntu:ubuntu and the synaptic bug looks to imply that it should be _apt user now.