Do you have any custom field validation using acf/validate_value?
What is happening and why are you seeing this error?
ACF attempts to validate the fields using AJAX before the form is submitted. During this validation an error occurs, usually a PHP error, but could any number of things. Whatever is happening is preventing ACF from sending the correct response to the validation AJAX request. When this happens ACF submits the form and fields are re-validated using PHP. An error in field input or in custom validation filters is causing the form submission to fail.
The solution is to track down the error that is causing the AJAX Request to fail. This generally means enabling error logging in WP and looking in the error log to find the error and correct it. https://wordpress.org/documentation/article/debugging-in-wordpress/
