Unable to update Plesk on a Debian/Ubuntu server: Failed to fetch URL. Some index files failed to download

Symptoms

• Plesk or system update fails with:

  E: Failed to fetch http://example.com/customrepo/php/ubuntu/dists/disco/main/binary-i386/Packages 404 Not Found
  E: Some index files failed to download. They have been ignored, or old ones used instead.

  ---

  W: Failed to fetch https://example.com/repo/dists/ubuntu-trusty/main/binary-i386/Packages HttpError404

  ---

  E: The repository 'http://ppa.launchpad.net/ondrej/php/ubuntu impish Release' does not have a Release file.
  N: Updating from such a repository can't be done securely, and is therefore disabled by default.

• A custom repository mentioned in the error message can be found in /etc/apt/sources.list.d/:

  # ls -l /etc/apt/sources.list.d/
  total 16
  -rw-r--r-- 1 root root 72 Jul 2 06:25 plesk-ext-nodejs.list
  -rw-r--r-- 1 root root 76 Jul 2 06:25 plesk-ext-ruby.list
  -rw-r--r-- 1 root root 76 Dec 15 05:12 plesk-migrator.list
  -rw-r--r-- 1 root root 97 Dec 15 05:09 plesk.list
  -rw-r--r-- 1 root root 76 Dec 15 05:12 customrepo.list

Cause

A 3rd-party repository mentioned in the error message is unreachable or misconfigured.

Resolution

1. Connect to the Plesk server via SSH.

2. Disable the affected repository:

   • If a 3rd-party repository is configured as a separate repo file:

     # mv /etc/apt/sources.list.d/customrepo.list /etc/apt/sources.list.d/customrepo.list.back

   • If a 3rd-party repository is configured in the file /etc/apt/sources.list, open this file in a text editor (for example, vi editor) and put a hash "#" symbol at the beginning of the line with a 3rd-party repository:

     # repo http://example.com/example_repo

3. Update APT packages list:

   # apt-get update

4. Update Plesk:

   # plesk installer update