Comment 0 for bug 1783267

linux-modules-extra-4.15.0-29-generic installs virtualbox guest modules that are newer than what virtualbox-guest-dkms-hwe provides (5.2.8 vs 5.1.34):

$ modinfo /lib/modules/4.15.0-29-generic/kernel/drivers/staging/vboxvideo/vboxvideo.ko
filename: /lib/modules/4.15.0-29-generic/kernel/drivers/staging/vboxvideo/vboxvideo.ko
license: GPL and additional rights
description: Oracle VM VirtualBox Graphics Card
author: Oracle Corporation
srcversion: 689EDF1E50DE15779B4F930
alias: pci:v000080EEd0000BEEFsv*sd*bc*sc*i*
depends: drm,drm_kms_helper,ttm
staging: Y
retpoline: Y
intree: Y
name: vboxvideo
vermagic: 4.15.0-29-generic SMP mod_unload
parm: modeset:Disable/Enable modesetting (int)

$ modinfo /lib/modules/4.15.0-29-generic/kernel/ubuntu/vbox/vboxsf/vboxsf.ko
filename: /lib/modules/4.15.0-29-generic/kernel/ubuntu/vbox/vboxsf/vboxsf.ko
version: 5.2.8_KernelUbuntu r120774
alias: fs-vboxsf
license: GPL
author: Oracle Corporation
description: Oracle VM VirtualBox VFS Module for Host File System Access
srcversion: AF89497125574B5225F9B6A
depends: vboxguest
retpoline: Y
intree: Y
name: vboxsf
vermagic: 4.15.0-29-generic SMP mod_unload
parm: follow_symlinks:Let host resolve symlinks rather than showing them (int)

$ modinfo /lib/modules/4.15.0-29-generic/kernel/ubuntu/vbox/vboxguest/vboxguest.ko
filename: /lib/modules/4.15.0-29-generic/kernel/ubuntu/vbox/vboxguest/vboxguest.ko
version: 5.2.8_KernelUbuntu r120774
license: GPL
description: Oracle VM VirtualBox Guest Additions for Linux Module
author: Oracle Corporation
srcversion: F2E5886E3B0544500D404A1
alias: pci:v000080EEd0000CAFEsv00000000sd00000000bc*sc*i*
depends:
retpoline: Y
intree: Y
name: vboxguest
vermagic: 4.15.0-29-generic SMP mod_unload

Thus installation with dkms fails (while attempting `apt-get install --reinstall virtualbox-guest-dkms-hwe`):

Building initial module for 4.15.0-29-generic
Done.

vboxguest:
Running module version sanity check.
Error! Module version 5.1.34_Ubuntu for vboxguest.ko
is not newer than what is already found in kernel 4.15.0-29-generic (5.2.8_KernelUbuntu).
You may override by specifying --force.

vboxsf.ko:
Running module version sanity check.
Error! Module version 5.1.34_Ubuntu for vboxsf.ko
is not newer than what is already found in kernel 4.15.0-29-generic (5.2.8_KernelUbuntu).
You may override by specifying --force.

vboxvideo.ko:
Running module version sanity check.
 - Original module
 - Installation
   - Installing to /lib/modules/4.15.0-29-generic/updates/dkms/

depmod....

DKMS: install completed.

My system does not start correctly with kernel 4.15 and I had to fall back to 4.13 which has working virtualbox guest modules.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: linux-modules-extra-4.15.0-29-generic 4.15.0-29.31~16.04.1
ProcVersionSignature: Ubuntu 4.13.0-45.50~16.04.1-generic 4.13.16
Uname: Linux 4.13.0-45-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: amd64
CurrentDesktop: MATE
Date: Tue Jul 24 08:33:57 2018
InstallationDate: Installed on 2015-02-04 (1265 days ago)
InstallationMedia: Ubuntu 14.04.1 LTS "Trusty Tahr" - Release amd64 (20140722.2)
SourcePackage: linux-hwe
UpgradeStatus: Upgraded to xenial on 2016-08-02 (720 days ago)