A couple of blogs appear to be using CloudFlare. If you look with your browser at blog, you get a message, "Checking Your Browser. This will take a few seconds." Then you can see the the blog fine. But TumblThree will give you error "Server returned error. 403 Forbidden" and stop.
Hasn't worked in earlier versions, not just 2.19.
Examples:
https://bethcullendsm-blog.tumblr.com/
https://bettertimes247.tumblr.com/