- actual Jetpack is installed
- actual "mobile-theme-featured-image" is installed
- in functions.php: add_theme_support( 'post-thumbnails' );
After uploading an image into the media library, an "HTTP error" appears. And the thumb in the media library is empty; because it has no "width" and "height" elements in the HTML code.
If I deactive this plugin, the http error still appears. But the thumbs in the media library are back.