The error appears when Contact Form 7 rejects a URL value during validation. It happens on the client side and stops the form from sending.
Validation requires a scheme such as http:// or https://. Missing scheme, spaces, or illegal characters cause the validator to flag the field as invalid.
The form also relies on the WordPress REST API endpoint. If the endpoint is unreachable because the REST API is disabled or rewrite rules are broken, the plugin interprets the response as an invalid URL.
Security modules that block requests to /wp-json/ also produce the same message. The request never reaches the server logic, so the plugin reports a failure.
Custom code that alters the wpcf7_validate_url filter or changes the form action might generate malformed URLs. The validator then rejects the submission.