package unattended-upgrades (not installed) failed to install/upgrade: subprocess installed post-removal script returned error exit status 1

[shabeeb]

shabeebk@shabeeb:~$ sudo apt --purge remove unattended-upgrades
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages will be REMOVED:
  unattended-upgrades*
0 upgraded, 0 newly installed, 1 to remove and 0 not upgraded.
11 not fully installed or removed.
After this operation, 315 kB disk space will be freed.
Do you want to continue? [Y/n] Y
(Reading database ... 255908 files and directories currently installed.)
Removing unattended-upgrades (0.90ubuntu0.9) ...
Purging configuration files for unattended-upgrades (0.90ubuntu0.9) ...
insserv: warning: script 'redis_6379' missing LSB tags and overrides
insserv: There is a loop between service monit and redis_6379 if stopped
insserv: loop involving service redis_6379 at depth 2
insserv: loop involving service monit at depth 1
insserv: Stopping redis_6379 depends on monit and therefore on system facility `$all' which can not be true!
insserv: exiting now without changing boot order!
update-rc.d: error: insserv rejected the script header
dpkg: error processing package unattended-upgrades (--purge):
 subprocess installed post-removal script returned error exit status 1
Processing triggers for man-db (2.7.5-1) ...
Errors were encountered while processing:
 unattended-upgrades
E: Sub-process /usr/bin/dpkg returned an error code (1)

ProblemType: Package
DistroRelease: Ubuntu 16.04
Package: unattended-upgrades (not installed)
ProcVersionSignature: Ubuntu 4.4.0-119.143-generic 4.4.114
Uname: Linux 4.4.0-119-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.15
Architecture: amd64
Date: Fri Apr 6 20:14:11 2018
ErrorMessage: subprocess installed post-removal script returned error exit status 1
InstallationDate: Installed on 2017-08-22 (227 days ago)
InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
RelatedPackageVersions:
 dpkg 1.18.4ubuntu1.4
 apt 1.2.26
SourcePackage: unattended-upgrades
Title: package unattended-upgrades (not installed) failed to install/upgrade: subprocess installed post-removal script returned error exit status 1
UpgradeStatus: No upgrade log present (probably fresh install)

[Balint Reczey (rbalint)]

Unattended-upgrades' postinst only triggered the problem which is in monit/redis:

...
Purging configuration files for unattended-upgrades (0.90ubuntu0.9) ...
insserv: warning: script 'redis_6379' missing LSB tags and overrides
insserv: There is a loop between service monit and redis_6379 if stopped
insserv: loop involving service redis_6379 at depth 2
insserv: loop involving service monit at depth 1
insserv: Stopping redis_6379 depends on monit and therefore on system facility `$all' which can not be true!
insserv: exiting now without changing boot order!
update-rc.d: error: insserv rejected the script header
dpkg: error processing package unattended-upgrades (--purge):
...

Please check the mentioned init.d scripts/services.