virtualbox.org

I am running VirtualBox on a ubuntu 16.04.2 LTS Host

Once I upgraded Virtual Box from 5.0.nn (sorry don't remember the exact version), none of my existing xubuntu 16.04 guest machines start, they all show status aborted with the following entries in the log:-
Shared clipboard: Initializing X11 clipboard backend
Shared clipboard: Failed to load the XFIXES extension
Shared clipboard: Failed to setup the termination mechanism
Shared clipboard: Initialization failed: VERR_NOT_SUPPORTED
Failed to initialize the shared clipboard

In the guest's settings the shared clipboard is not selected

I've even tried creating a new xubuntu guest from scratch, which is fine until I install the guest additions, then the guest machine aborts.
Guest Additions is version 5.1.22_115126

Please post a VM log file. With the VM fully shut down, right click and "Show Log" in the GUI, save "VBox.log" (no other file) to a zip, and attach the zip here.

Log File attached

As I suspected :-

00:00:15.670494 VMMDev: Guest Additions information report: Version 5.0.18 r106667 '5.0.18_Ubuntu'

The Guest Additions currently installed are not from VirtualBox, they are from someone's fork. Please purge the third party fork GAs and install the correct ones. Please not that you can NOT simply upgrade the GAs, since they are not seen as the same package - and you definitely don't want two incompatible GA packages fighting one another.

OK - Thanks

How can I purge the GA's, if I can't start the VM?

Also, the GA's were installed from within VirtualBox so why are they from a fork?

So, how do I install GA's not from a fork?

Eh? I missed the part where the guest OS failed to start (the VM does start, or who is delivering the error messages?), I thought you just mean that features such as shared clipboard were not available. Since the GAs are just optional drivers I don't see how they can stop the guest OS from booting up, but I'll leave that discussion for Linux experts.

Even when I had VirtualBox 5.1.22 & GA v 5.1.22, the guest aborted on start.

I've resolved this for now by removing VirtualBox 5.1.22 (including the GA's) and reinstalling v5.0.36 from the debian repositories.

I seem to remember a similar problem when I previously tried to upgrade VirtualBox from the virtual box website, rather than from the repository

Well, naturally. Your problem is that you are using the repo GAs. Since you have unsupported and interfering GAs installed you can't expect them to work with VirtualBox.