Unable to install Plesk updates on Ubuntu: E: dpkg was interrupted, you must manually run 'dpkg --configure -a' to correct the problem

Symptoms

1. Unable to install Plesk updates on Ubuntu 14.04 or Ubuntu 16.04. The following error is displayed in the Plesk interface:

   PLESK_ERROR: E: dpkg was interrupted, you must manually run 'dpkg --configure -a' to correct the problem.

2. The following explanation can be found in /var/log/plesk/systemupdatestool.log

   # cat /var/log/plesk/systemupdatestool.log | tail -n3
   2017-04-02 06:22:15 INFO: pum is called with arguments: ['--list', '--repo-info', '--json']
   2017-04-02 06:22:15 ERROR: Previous Plesk installer call finished unsuccessfully. PUM will not perform any update operations until Plesk installation is fixed.
   2017-04-02 06:22:15 ERROR: Exited with returncode 50.

3. dpkg --configure -a command hangs at restarting AppArmor and after that server reboot is required:

   CONFIG_TEXT: Setting up apparmor (2.10.95-0ubuntu2.6) ...
   update-rc.d: warning: start and stop actions are no longer supported; falling back to defaults

Cause

Bug in the Ubuntu kernel - #1579135: AppArmor profile reloading causes an intermittent kernel BUG

Resolution

Warning: We strongly recommend to backup the whole server before applying the steps below. If your Plesk is running inside a virtual container, contact your service provider to create a backup and in order to verify if the kernel version is supported.

1. Make sure that error is the same as in symptoms.
2. Log in to the server via SSH.
3. Temporarily remove AppArmor post-install scripts in order to prevent AppArmor from reloading:

   # mv /var/lib/dpkg/info/apparmor.postinst ~/
   # mv /var/lib/dpkg/info/apparmor-profiles.postinst ~/

4. Disable AppArmor:

   # service apparmor stop
   # service apparmor teardown
   # update-rc.d -f apparmor remove

5. Run dpkg --configure -a to fix the initial error:

   # dpkg --configure -a

6. Upgrade Ubuntu kernel to version 4.4.0-38.57 or newer and reboot the server. For example, for version 4.4.0-72:

   # apt update
   # apt install linux-image-4.4.0-72-generic
   # sudo reboot

7. Move back AppArmor post-install scripts and reconfigure apparmor-profiles package:

   # mv ~/apparmor.postinst /var/lib/dpkg/info/
   # mv ~/apparmor-profiles.postinst /var/lib/dpkg/info/
   # dpkg-reconfigure apparmor
   # dpkg-reconfigure apparmor-profiles

8. Start AppArmor:

   # service apparmor start