Shared directory with Guest Ubuntu has become empty: a problem of virtualbox guest addition?
Just solved the shared directory problem (showing the content of the shared directory), by installing the guest addition under gnome terminal of the guest Ubuntu:

cd /media/t/VBOXADDITIONS_5.0.16_105871/

sudo ./VBoxLinuxAdditions.run 


See "Installing Guest Additions from a Terminal"  in https://askubuntu.com/a/22745/1471.

Questions:


Why did the above command line solution succeed in showing the content of the shared directory.
What is the cause of my problem in my post here?
I still get the same error when clicking Devices -> Insert Guest Addition CD Image as in my post here.