Tested this now and the image downloaded smoothly, thats mean you have an internal issue you have to fix.
Maybe try to download the image over torrent (torrent client example can be transmission):
https://download.whonix.org/ova/17.2.3.7/Whonix-Xfce-17.2.3.7.ova.torrent
For .asc and .sig:
https://download.whonix.org/ova/17.2.3.7/Whonix-Xfce-17.2.3.7.ova.torrent.asc
https://download.whonix.org/ova/17.2.3.7/Whonix-Xfce-17.2.3.7.ova.torrent.sig