I uninstalled all WordPress Plugins (except CiviCRM). Then, I uninstalled all CiviCRM Extensions and Components.
Only when I disable the CiviCRM plugin does this error disappear:
CiviCRM 5.22.1 / 5.23.4
The REST API encountered an 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 10000 milliseconds with 0 bytes received (http_request_failed) 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: cURL error 28: Operation timed out after 10000 milliseconds with 0 bytes received (http_request_failed)