Can't install Plesk updates: Conflicting values set for option Signed-By regarding source

Symptoms

• Plesk update fails to open or install

  ERR_CONNECTION_RESET

  ERR_CONNECTION_REFUSED

  PR_CONNECT_RESET_ERROR

• These errors are logged to /var/log/plesk/install/autoinstaller3.log

  Synchronizing the Debian APT package index files...
  E: Conflicting values set for option Signed-By regarding source https://repos.insights.digitalocean.com/apt/do-agent/ main: /usr/
  share/keyrings/digitalocean-agent-keyring.gpg !=
  E: The list of sources could not be read.
  Warning: there are errors in 'apt-get update'

• Manually running apt-get update fails

  # apt-get update
  N: Ignoring file 'plesk.list.ai_back' in directory '/etc/apt/sources.list.d/' as it has an invalid filename extension
  E: Conflicting values set for option Signed-By regarding source https://repos.insights.digitalocean.com/apt/do-agent/ main: /usr/share/keyrings/digitalocean-agent-keyring.gpg !=
  E: The list of sources could not be read.

Cause

This error happens because there's an invalid repository file, which prevents the system from running any updates.

Resolution

To fix this problem, remove the repository file from the error.

1. Connect to the server via SSH

2. Move the incorrect repository file (in the example above, from DigitalOcean).

   # mv /etc/apt/source.list.d/digitalocean-agent.list /root/digitalocean-agent.list

3. Open the Plesk installer