Ubuntu 16.04 update manager error

In the command-line type this content:

wget -q http://download.virtualbox.org/virtualbox/debian/oracle_vbox_2016.asc -O- | sudo apt-key add -

You need to remove the old keys left over from before the upgrade. Do:

sudo apt-key list

Find the offending key (near "virtualbox", after "pub", xxx/KEYHERE) and do:

sudo apt-key del KEYHERE

Then go to https://www.virtualbox.org/wiki/Linux_Downloads and follow the instructions at "Debian-based Linux distributions"

You obviously did a system upgrade without having removed the external repositories before. The easiest way to solve the problem would be to remove all external repositories and delete all related public keys. After having done this, update the repositories (sudo apt-get update) and check whether everything is fixed now ... and when no error occurs anymore, you can add new external repositories, but carefully check whether these repositories are "matching Xenial" ...

Depending on which repositories you add, a "weak signature" warning still may appear when updating the repositories - but you can ignore it, because it is just an informational message.

gpg: Signature by key ... uses weak digest algorithm (SHA1)