Form not working. “Something went wrong.”

We have very simple, one field, subscription form.
On submit the form makes an Ajax request to: /wp-json/contact-form-7/v1/contact-forms/3684/feedback
But returns: {“into”:”#wpcf7-f3684-o2″,”status”:”mail_failed”,”message”:”Something went wrong. Please click the Subscribe button once again.”,”posted_data_hash”:”2cf226fa7480e77358f523889e37ac9e”}

What may be the reason? And how to debug the error?

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