WordPress Site Health error on Plesk server: The REST API encountered en error / Your site could not complete a loopback request.

Symptoms

• The following error is returned when opening Domains > example.com > WordPress admin dashboard > Tools > Site Health:

  CONFIG_TEXT: The REST API encountered en error
  
  The REST API is one way WordPress, and other applications, communicate with the server. One example is the block editor screen, which relies on this to display, and save, your posts and pages.
  The REST API request failed due to an error.Error: [] cURL error 28: Operation timed out after 10011 milliseconds with 0 bytes received

  CONFIG_TEXT: Your site could not complete a loopback request
  
  Loopback requests are used to run scheduled events, and are also used by the built-in editors for themes and plugins to verify code stability.
  The loopback request to your site failed, this means features relying on them are not currently working as expected.Error encountered: (0) cURL error 28: Operation timed out after 10005 milliseconds with 0 bytes received

• Enabling loopback as described in the article does not help.

Cause

WordPress known internal issue e.g. REST API Failure.

The issue can also be caused by some plugins.

Resolution

Use one of the following resolutions:

1. Log in to Plesk, go to WordPress > example.com > Plugins tab and disable all plugins for the affected domain. In case the issue is fixed, enable them back one by one to find the problematic plugin.

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2. Wait for the issue to be fixed on WordPress side or contact WordPress support for further assistance.