I have the same issue today on our Staging site after updating CF7 from 5.1.9 to 5.2.
Empty bordered box (.wpcf7-response-output) now shows on the page below the form’s send button before form is filled or submitted; no close button in my case.
Deactivated all other plugins (except woocommerce and wpbakery page builder). Issue remains.
The form sends & works properly and the messages do work when supposed to as well (i.e. error messages and our confirmation message after submitting).
For time being, I rolled back to CF7 5.1.9 and the issue’s gone.
