Failure to respond to the server’s data requests in a timely manner causes cURL error 28 in WordPress.
WordPress uses REST API (a programming technique) to send and receive data requests. If these requests time out, then you will see a critical issue in the Site Health report with the title ‘The REST API encountered an error’.
Expanding the error will show you further details, including the error message:
‘Error: cURL error 28: Operation timed out after x milliseconds with x bytes received (http_request_failed)’
You may also see another related issue with the title ‘Your site could not complete a loopback request’. It will have a similar error message with the following description.
‘The loopback request to your site failed, this means features relying on them are not currently working as expected.’