This post saved me again installing the extension packs 
For the 3.7.14 version seems like the exact (new) command for them is:
sudo VBoxManage extpack install --replace Oracle_VM_VirtualBox_Extension_Pack-6.0.12.vbox-extpack --accept-license=27a0956940654b0accf4d79692078bd496d9f062e4ed3da69e5421cba8d1e444
I don't understand why the GUI installer just don't works... but happy to have it working (after a few more 2-3 errors/tests)
