There’s at least some minor problem with that server’s SSL/TLS configuration, which will result in issues with PHP being able to connect to that server and download anything. This will cause problems when trying to download the manifest itself, and also when trying to retrieve any images or other resources linked in the manifest, since they’re on the same server.
I’m not 100% certain this is the ultimate cause of your problems (I was able to import using the “From File” option, at least), but it’s a possibility. The issue is something the IT staff or server administrators for that server would have to work out: basically, the server is not correctly sending the “intermediate” security certificate it should be sending.