Unable to install or update Plesk license: KA Request Failure desc: cURL cannot communicate with license server

Symptoms

• Unable to install or update license in Plesk by going to Tools & Settings > License Information > Install Key, because the following error shows up in the Plesk GUI:

  Could not update the license. Make sure that connections to the license server ka.plesk.com on TCP port 443 are not blocked. See how to do so.

• Errors that are similar to the following are recorded in /var/log/plesk/panel.log:

  [2023-04-09 13:36:05.641] 2629852:6432f779458e7 ERR [panel] KA Request Failure code: 2 Network failure
  [2023-04-09 13:36:05.641] 2629852:6432f779458e7 ERR [panel] KA Request Failure desc: cURL cannot communicate with license server https://id-00.kaid.plesk.com:443/ (127.0.0.1): SSL connect error(35)
  cURL cannot communicate with license server https://id-00.kaid.plesk.com:443/ (): Couldn't resolve host name(6)
  [2023-04-09 13:36:05.641] 2629852:6432f779458e7 ERR [panel] KA Request Failure additional information: cURL verbose output:
  About to connect() to id-00.kaid.plesk.com port 443 (#0)
  Trying 127.0.0.1…
  Connected to id-00.kaid.plesk.com (127.0.0.1) port 443 (#0)
  Initializing NSS with certpath: sql:/etc/pki/nssdb
  NSS error -12263 (SSLERRORRXRECORDTOO_LONG)
  SSL received a record that exceeded the maximum permissible length.
  Closing connection 0

  cURL verbose output:
  Could not resolve host: id-00.kaid.plesk.com; Name or service not known
  Closing connection 1

• After license activation, wrong Key Number is shown is Plesk > Tools & Settings > License Information.

Cause

Plesk KA (Key Adminstrator) server IPs are defined and incorrectly assigned to localhost IPs (or other incorrect IPs) in the /etc/hosts file of the server, for example:

# root @server in ~: cat /etc/hosts
203.0.113.2 ka.plesk.com
127.0.0.1 id-b1.kaid.plesk.com ka.plesk.com leika.plesk.com id-00.kaid.plesk.com id-01.kaid.plesk.com

Resolution

1. Connect to the server via SSH

2. Edit the /etc/hosts file with your favorite command-line text editor

3. Remove the KA Server lines entirely (or comment them by placing a # symbol at the start of each such line)

4. Save the changes and close the file

Afterwards, you should be able to repeat the Plesk license installation or update action via the Plesk GUI without encountering errors.