Google "noCaptcha" is not showing fallback image

For some reason, when the regular botchell of the new Google reCaptcha noCaptcha fails, the backup image is not displayed on my joomla site. Instead, I just get a popup speech with alt text instead of an image. Strange because it runs on a different site on the same server ... I also get a 400 error (NetworkError: 400 Bad Request - https://www.google.com/recaptcha/api2/payload?c=03AZJ_uuqdT ... ). What could be causing this? And how can I fix this problem?