This blog is hosted on a NGINX Server with some recommended NGINX security rules enabled. It also does not use .htaccess so out of the box the ActivityHub Plugin does not work. Here is a little code snippet you can add to your NGINX configuration that did the trick for me.

# Add Activity Pub Config
location = /.well-known/webfinger {
    auth_basic off;
    allow all;
    default_type text/plain;
    try_files $uri $uri/ /index.php?$args;
    break;
}

If you want to test if these changes have worked go here.