Validation error occurred. No specification on the error(s)

The form https://rawsons.nu/promo/test-till-tavling/ has a number of fields available, shown based on settings in custom fields for each page in WP. There are three forms on the site, where one is working, and two aren’t.

The only difference is that the working one is not using AJAX, instead it redirects via PHP to a specific landing page.

On the AJAX-enabled forms, the message “Validation errors occurred” (in Swedish) appear when submitting the form.

One of the AJAX forms has this [textarea* tavling x4 class:form-control] field included, the other one doesn’t.

There’s also a tag in the forms, that’s just replaced by text (not a form field: [tavlingstext] is replaced by this function:

function tavlingstext_shortcode($tag) {
	global $post;
	return get_field('tavlingstext', $post->ID);
}
wpcf7_add_shortcode('tavlingstext', 'tavlingstext_shortcode', true);

(The get_field function is from Advanced Custom Fields and gets a custom field value from a post)

Please help figure out where the error comes from. No field is highlighted, and no output gives any clue.

https://www.ads-software.com/plugins/contact-form-7/