virtualbox.org

I have just created a new Ubuntu 14.04.4 64 bit guest on a Windows 10 host, installed the Guest additions, and rebooted the guest OS, but the Full Screen view does not appear to work.
The guest OS appears to be stuck at 640x480 resolution, and is too small to be usable. When I click View-->Full screen, the guest OS just appears as a small 640x480 area in the centre of a black screen (see attached screen capture).
My Virtualbox version is: 5.1.4r110228, and the guest additions I installed were the one supplied with this Virtualbox version.
Also, the Seamless Mode, and Auto-resize Guest Display option from the View menu for the guest OS are greyed out.
Does anyone know how to get my guest OS to display using the full screen?
I have posted my Virtualbox log file for further information. Note I have had to post this in 2 parts, as it is too large for a single attachment.

VirtualBox VM 5.1.4 r110228 win.amd64 (Aug 16 2016 19:17:01) release log
00:00:39.361877 VMMDev: Guest Additions information report: Version 5.0.0 r101573 '5.0.0_Ubuntu'

I am afraid you do not have the right additions installed but rather the ones from the latest Ubuntu package in their repository.

Remove those and install the right ones from your installed version of VirtualBox.

Note: using the device tab and clicking insert guest additions do not actually install them. You need to finish the install from terminal.

I installed the guest additions from the Device tab and completed the installation in the terminal that opened. The Guest additions CD image is reported as VBOXADDITIONS_5.1.4_110228, so I don't know why the wrong version is listed in the log.
I have tried repeating the above from the guest OS, but I still have the same problem with full screen mode not working.
Can anyone let me know how to remove completely the installed guest additions, and then make sure the correct ones get installed?
I have tried sudo apt-get purge virtualbox* from a terminal, which appeared to execute ok. I then rebooted the guest OS, but still the Virtualbox log shows:
VMMDev: Guest Additions information report: Version 5.0.0 r101573 '5.0.0_Ubuntu'