The download link points to https://download.fedoraproject.org/pub/fedora/linux/releases/test/23_Alpha/Cloud/x86_64/Images/Fedora-Cloud-Base-Vagrant-23_Alpha-20150805.x86_64.vagrant-libvirt.box which is a 404.
The correct link should be: https://download.fedoraproject.org/pub/fedora/linux/releases/test/23_Alpha/Cloud/x86_64/Images/Fedora-Cloud-Base-Vagrant-23_Alpha-20150806.2.x86_64.vagrant-virtualbox.box
Sorry, the corrected link was wrong above, should be: https://download.fedoraproject.org/pub/fedora/linux/releases/test/23_Alpha/Cloud/x86_64/Images/Fedora-Cloud-Base-Vagrant-23_Alpha-20150806.2.x86_64.vagrant-libvirt.box
Seems this affects all the Base images, due to a different build date. I pushed rev. 31bab8e as a fix, this will work but we need to revert this to keep the pages maintainable.
Hopefully we won't have different build dates for Beta.
Thank you for letting us know.