virtualbox.org

recently (probably after update) Ubuntu 16 doesn't reboot properly. A VirtualBox stop job is running for LSB, for 5 minutes. I can only power off and restart.

When if fails, and the VM powered-off, right-click on the VirtualBox Manager and select "Show Log". Save the first "VBox.log" only, ZIP it and attach it in your response (see the "Upload attachment" tab below the reply form).

Thanks, log attached

VirtualBox VM 5.1.6_Ubuntu r110634 linux.amd64 (Sep 14 2016 06:43:10) release log

You're running an older version of VirtualBox (we're now to 5.1.14), but that's not a problem. What can be a problem is that you're using the distro's fork version of VirtualBox. You can either ask in their forums for help, or completely remove/uninstall/delete/purge their version and install the official version from the Downloads section of VirtualBox (https://www.virtualbox.org/wiki/Downloads).

Plus you are running newer versions of the guest additions than the version of VBox you have installed. This has been known to cause all kinds of issues.