18.04 VM: Upgrading Kernel wont start after upgrading kernel

[undecidable]

Created a new VM and clean-installed Xubuntu 18.04.
Did all updates except kernel updates, and it rebooted fine.

But then after doing the kernel updates in the VM
the system fails to boot
even if I try to boot with the old kernel,
and even in recovery mode on the old kernel.

Setting the boot sequence to non-quiet, the boot sequence stops at
the following message:

Booting SMP Configuration

though that might not necessarily be the problem.
All messages prior to it seem unremarkable.

I suspect the microcode updates,
but they work fine on the Kubuntu 14.04 VMs,
and anyway there is no way to disable installing them.

The kernel update which is being done:

The following NEW packages will be installed:
   amd64-microcode (3.20180524.1~ubuntu0.18.04.1)
   intel-microcode (3.20180425.1~ubuntu0.18.04.1)
   iucode-tool (2.3.1-1)
   linux-headers-4.15.0-23 (4.15.0-23.25)
   linux-headers-4.15.0-23-generic (4.15.0-23.25)
   linux-image-4.15.0-23-generic (4.15.0-23.25)
   linux-modules-4.15.0-23-generic (4.15.0-23.25)
   linux-modules-extra-4.15.0-23-generic (4.15.0-23.25)
The following packages will be upgraded:
   linux-generic (4.15.0.20.23 => 4.15.0.23.25)
   linux-headers-generic (4.15.0.20.23 => 4.15.0.23.25)
   linux-image-generic (4.15.0.20.23 => 4.15.0.23.25)

Information about the system:
Host
Kubuntu 14.04
Linux ox1lx1 4.4.0-112-generic #135~14.04.1-Ubuntu SMP Tue Jan 23 20:41:48 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
Virtualbox Version: 4.3.36-dfsg-1+deb8u1ubuntu1.14.04.1

Guest
Xubuntu 18.04
Linux Xb80 4.15.0-20-generic #21-Ubuntu SMP Tue Apr 24 06:16:15 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

I have 4 Kubuntu 14.04 guests, and W7 and Wxp guests with no problems
so the basic setup is stable.

I need to solve this as it is about time I upgraded all systems from 14.04 to 18.04.
Any ideas?

[halogen2]

Your VirtualBox version is very old. Try upgrading your VirtualBox to the latest version from the official website.

[undecidable]

thanks, yes, I assumed this was the case.
but was hoping for a bit of explanation or workaround.
eg I had a similar problem installing the virtualboxguestadditions.iso
but worked around it by installing
virtualbox-guest-x11 virtualbox-guest-dkms & virtualbox-guest-utils
from the repos.

As for getting the "official" version from Oracle,
will do it soon after the universe reaches 10^-43 deg Kelvin.
I use the Ubuntu repos.

In any event I have continued testing Xb18.04 in the VM on the 14.04 host
just without upgrading the kernel,
and in the next few days will install Xb18.04 on a host.
So likely the problem will then disappear.

[undecidable]

Just to confirm and close this,
as halogen2 suggested, it was a virtualbox version compatibility issue.

I created a new Xubuntu 18.04 host,
shifted the Xubuntu 18.04 guest to it,
and the kernel update problem with the Xubuntu 18.04 guest disappeared.