Could you kindly copy here the URL which is giving 404? Probably it’s a file system permissions problem or a security plugin which is blocking that url.
Debugging shows https://widgets.wp.com/client/boot/stylesheets/shared/reset.scss
is not found in https://widgets.wp.com/notifications/build.css.map?4aa3197ad83fef9edfbc434bc6b189
but listed.
A 404 means the file which generates the preview (inside the plugin folder) cannot be called directly for some reason. You should ask the provider: it could be a file permission problem, or an htaccess configured to bock that file.