I tried setting up a Xenial vagrant guest on my Mac yesterday, using the official ubuntu/boxes/xenial64 and ran into various problems.
Some are mentioned here:
But one of the most relevant problems is that the official box does not use the conventional vagrant/vagrant user/pw combination as the default user. Rather it uses Ubuntu’s original default user “ubuntu”. This is a problem for many existing scripts and developed habits.
This problem is addressed here already: Bug report – ubuntu user vs conventional vagrant user
As there is currently no reaction from the official Ubuntu box creators, a saving comment recommends using bento/ubuntu-16.04, which are configured correctly, according to vagrant standards.