[ubuntu] Problem on shutdown PC

Hi all,

Host Ubuntu 16.04
Oracle VirtualBox

On shutdown following warning popup
The PC hangs. I have to turn it off manually.

In one occasion on rebooting PC, above warning also popup. It hung a while and then rebooted.

However I can start Virtualbox and VMs without problem. Please advise where shall I check and how to fix the problem.

TIA

Regards
satimis