Unable to secure Plesk panel with Let’s Encrypt: Invalid port in redirect target. Only ports 80 and 443 are supported, not 8443

Symptoms

• Unable to secure Plesk panel with Let's Encrypt.
  The following error appears

  Invalid port in redirect target. Only ports 80 and 443 are supported, not 8443

• A hostname example.com used to access Plesk panel is not served by Plesk and absent in the domains list.

• The following can be found in /var/www/vhosts/default/htdocs/.htaccess file:

  Redirect / https://example.com:8443/login_up.php3

Cause

Custom redirects configured for the Plesk hostname

Resolution

1. Connect to the server via SSH

2. Remove redirect to Plesk hostname from /var/www/vhosts/default/htdocs/.htaccess

3. Configure Plesk accessible over a hostname referring the following article:
   How to make Plesk interface accessible over a hostname without entering the port number on a Linux server