Just an update because I was encountering this myself and stumbled on the forum posts:
It seems the URL listed for the download attempts to access a restricted directory. Remove it from the URL path and you'll be able to download the image. Also, you'll notice the image name includes the timestamp 20190321 (March 21, 2019) and the support page lists the latest image as April 4, 2019. I wasn't able to access that image by changing the timestamp in the name, I get a 404 (not found) error.
In summary, change the URL to the image from this:
That should do the trick. Some web developer somewhere seems to have goofed or the image really isn't supposed to be available to download yet. Either way, anything under that restricted path will return access denied errors.