A bug has been introduced into the netboot images of Ubuntu 16.04 causing it to halt during install.
Please see the following URLs for more information:
We are monitoring this and hopefully it will be resolved very soon. In the mean time Ubuntu 18.04 LTS works just fine.
Update 28.03.2018: This has still not been resolved. Will close this for now. When the issue is resolved, we will enable 16.04 again.