Problem upgrading from 14.04 to 16.04

Hi,

I have a 14.04.5 that I'm ready to lift to 16.04. But when running do-release-upgrade, at the first "config-file question", do I want to keep current or install new etc, when I press a key it goes to "Login incorrect. Give root password for maintenance (or type Control-D to continue)" and if I try to type the root password o press Control-D it just prompts again. If I hold Enter it eventually moves on but at the next config-file question it happens again. Any idea how to fix? It's a VM and I have reverted back to a working 14.04 snapshot so no panic, but I need to upgrade eventually.