Plesk automatic update warning: Apt cache fetch failed. Try to run the `apt-get update` command.

Symptoms

• On a Debian-based server, Plesk automatic updates return errors that are similar to the following whenever they run:
  

  PLESK_INFO: 2024-01-01 00:00:00 ERROR: Apt cache fetch failed. Try to run the `apt-get update` command.
  2024-01-01 00:00:00 ERROR:
  2024-01-01 00:00:00 ERROR: Exited with returncode 1.

• A custom repository called mariadb.list is configured in the /etc/apt/sources.list.d directory.
• The /var/log/plesk/install/autoinstaller3.log has the following error:
  

  PLESK_INFO: Err:01 https://dlm.mariadb.com/repo/mariadb-server/10.11/repo/ubuntu focal InRelease
  500 Internal Server Error [IP: 203.0.113.2]
  Reading package lists...
  W: Failed to fetch https://dlm.mariadb.com/repo/mariadb-server/10.11/repo/ubuntu/dists/focal/InRelease 500 Internal Server Error [IP: 203.0.113.2]

• Running apt-getupdate lists the repositories successfully.

Cause

The mariadb.list file is created when a MariaDB upgrade is performed either via the Plesk panel or manually by calling the mariadb_repo_setup script. This list file that adds the CDN repository dlm.mariadb.com by default, which was not reachable at the time the Plesk update took place.

Resolution

Since the dlm.mariadb.com is known to cause issues, you should disable the repositories in the /etc/apt/sources.list.d/mariadb.list (you can comment them out) and add an alternative ones based on the MariaDB version and exact server OS you are using from here:

https://mariadb.org/download/?t=repo-config