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Suspect this issue is happening when the site is accessed via https://example.com instead of https://www.example.com, or when moving from one domain to another midway through a form submission.

See also this similar SE question. This text is in the error message you've pasted:


  Site Administrators: This error may indicate that users are accessing this page using a domain or URL other than the configured Base URL. EXAMPLE: Base URL is http://example.org, but some users are accessing the page via http://www.example.org or a domain alias like http://myotherexample.org


Make sure both WordPress and CiviCRM have a matching URL configured - CiviCRM in civicrm.settings.php, WordPress in Settings ⇒ General admin page (ref WP Codex: Changing the Site URL). Ideally, make sure that anyone landing on alternate URLs is redirected immediately to the canonical hostname and protocol.

The .htaccess snippet in this question addresses consistent protocol but not hostname.