There was a lot of strange problems with ubuntu/xenial64, and in https://github.com/mitchellh/vagrant/issues/6616 there is a mention by Seth Vargo (employee of Hashicorp)
The ubuntu/xenial64 box is built wrong and horribly broken. Please note that "ubuntu" is the name of a user, not a representation of a canonical source for ubuntu images. Please try bento/ubuntu-16.04 instead. Thanks.
These errors included following:
rejecting i/o to offline deviceThis happened almost everytime after heavier I/O operations, for example after loading Docker images.
stderr: Inappropriate ioctl for deviceI think that this happened when Vagrant tried to setup network interfaces, mainly "enp0s8".
So just use bento/ubuntu-16.04