Investigating further, it is quite possible the problem appeared in APT 1.2.15. Timeline of install and updates on my machine:
* 2016-07-19: U apt:amd64 1.2.10ubuntu1 -> 1.2.12~ubuntu16.04.1 # OS install
* 2016-09-26: I update-notifier-common:amd64 3.168.1 (automatic)
* 2016-09-26: I ttf-mscorefonts-installer:amd64 3.4+nmu1ubuntu2 # no problem
* 2016-11-04: U update-notifier-common:amd64 3.168.1 -> 3.168.2 # no problem
* 2016-11-14: U apt:amd64 1.2.12~ubuntu16.04.1 -> 1.2.15
* 2016-12-14: U apt:amd64 1.2.15 -> 1.2.15ubuntu0.2
* 2017-01-06: U update-notifier-common:amd64 3.168.2 -> 3.168.3 # PROBLEM
Between the time it ran with no problem on 2016-11-04 and the time the problem appeared, APT received two updates. According to the ChangeLog, the second one was a rather focused security fix. It is more likely that the first one is at fault, this it contains many minor changes.
Investigating further, it is quite possible the problem appeared in APT 1.2.15. Timeline of install and updates on my machine:
* 2016-07-19: U apt:amd64 1.2.10ubuntu1 -> 1.2.12~ ubuntu16. 04.1 # OS install notifier- common: amd64 3.168.1 (automatic) -installer: amd64 3.4+nmu1ubuntu2 # no problem notifier- common: amd64 3.168.1 -> 3.168.2 # no problem ubuntu16. 04.1 -> 1.2.15 notifier- common: amd64 3.168.2 -> 3.168.3 # PROBLEM
* 2016-09-26: I update-
* 2016-09-26: I ttf-mscorefonts
* 2016-11-04: U update-
* 2016-11-14: U apt:amd64 1.2.12~
* 2016-12-14: U apt:amd64 1.2.15 -> 1.2.15ubuntu0.2
* 2017-01-06: U update-
Between the time it ran with no problem on 2016-11-04 and the time the problem appeared, APT received two updates. According to the ChangeLog, the second one was a rather focused security fix. It is more likely that the first one is at fault, this it contains many minor changes.