A server shows the error in Plesk 360: Connection error

Symptoms

• A new Plesk server can not be added to Plesk 360 with one of the following errors:

  PLESK_ERROR: The connection to the server server.hostname.com cannot be established correctly. Plesk version is not supported (below 18.0.37) or Plesk is misconfigured.

  ---

  PLESK_ERROR: The network connection to the server timed out

  ---

  PLESK_ERROR: Unknown error: connect ECONNREFUSED 213.0.1.2:443

• The following error appears under Server state in Plesk 360 for an already added server:

  PLESK_ERROR: Connection error

• The following message appears under Plesk 360 > Server > server.hostname.com:

  PLESK_ERROR: The connection to the server server.hostname.com cannot be established correctly. The server is configured incorrectly or you have entered an incorrect address.
  
  Please fix the error and press the button below to restore the connection

Cause

Access to the server is blocked in one of the following ways:

• a firewall rule blocks connections
• Fail2Ban bans the IP addresses
• IP addresses of Plesk 360 are filtered by IP Restrictions for Administrative access
• the API filtration rules are configured

Resolution

Allow access for the necessary IP addresses (52.51.23.204, 52.213.169.7, 34.254.37.129):

• in a firewall (if there is the Plesk Firewall check this article)
• in Fail2Ban (add the IP addresses to Trusted IP Addresses)
• in Plesk > Tools & Settings > IP Access Restriction Management (the instructions might be found here)
• in the section [api] inside the file panel.ini (the details are here)