Screen resolution reduced on Ubuntu Guest in Virtualbox with Guest Additions already installed

Question

I have been using Ubuntu 18.04 on Virtualbox 6.1 on a Windows 10 host for months now. I installed Guest Additions on initial install, and everything was fine, including screen resolution scaling up. Yesterday, with no obvious change on my part, the Guest has reverted to 800x600px resolution. I am able to change that from inside the guest, but the change spontaneously reverts itself periodically to 800x600px.

Guest Additions is already installed and functional in every other way I can think to test, but resolution is broken.

After this happened I updated Virtualbox to the latest version, but this did not change anything.

Has anyone encountered this before? All my searches related to guest screen resolution just bring me to pages telling me to install Guest Additions, which is already done and was working until yesterday.

I could nuke it and start over, but I'm hoping there's a less annoying way.

Answer 1

If you try to reinstall Guest Additions and you get an error

VirtualBox Guest Additions: Starting.
VirtualBox Guest Additions: Building the VirtualBox Guest Additions kernel 
modules.  This may take a while.
VirtualBox Guest Additions: To build modules for other installed kernels, run
VirtualBox Guest Additions:   /sbin/rcvboxadd quicksetup <version>
VirtualBox Guest Additions: or
VirtualBox Guest Additions:   /sbin/rcvboxadd quicksetup all
VirtualBox Guest Additions: Building the modules for kernel 5.8.0-38-generic.

VirtualBox Guest Additions: Look at /var/log/vboxadd-setup.log to find out what 
went wrong

Probably your linux kernel was updated as it was in my case. So I had to rollback linux kernel from 5.8.0-38-generic to 5.4.0-62-generic.

Now everything works fine.

$ uname -a
Linux z-VirtualBox 5.4.0-62-generic #70-Ubuntu SMP Tue Jan 12 12:45:47 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

$ dpkg -l | grep linux-image
rc  linux-image-5.4.0-26-generic               5.4.0-26.30                         amd64        Signed kernel image generic
rc  linux-image-5.4.0-39-generic               5.4.0-39.43                         amd64        Signed kernel image generic
rc  linux-image-5.4.0-40-generic               5.4.0-40.44                         amd64        Signed kernel image generic
rc  linux-image-5.4.0-42-generic               5.4.0-42.46                         amd64        Signed kernel image generic
rc  linux-image-5.4.0-45-generic               5.4.0-45.49                         amd64        Signed kernel image generic
rc  linux-image-5.4.0-47-generic               5.4.0-47.51                         amd64        Signed kernel image generic
rc  linux-image-5.4.0-48-generic               5.4.0-48.52                         amd64        Signed kernel image generic
rc  linux-image-5.4.0-51-generic               5.4.0-51.56                         amd64        Signed kernel image generic
rc  linux-image-5.4.0-52-generic               5.4.0-52.57                         amd64        Signed kernel image generic
rc  linux-image-5.4.0-53-generic               5.4.0-53.59                         amd64        Signed kernel image generic
rc  linux-image-5.4.0-54-generic               5.4.0-54.60                         amd64        Signed kernel image generic
rc  linux-image-5.4.0-56-generic               5.4.0-56.62                         amd64        Signed kernel image generic
rc  linux-image-5.4.0-58-generic               5.4.0-58.64                         amd64        Signed kernel image generic
rc  linux-image-5.4.0-59-generic               5.4.0-59.65                         amd64        Signed kernel image generic
ii  linux-image-5.4.0-62-generic               5.4.0-62.70                         amd64        Signed kernel image generic
ii  linux-image-generic                        5.4.0.62.65                         amd64        Generic Linux kernel image

It didn't go very smoothly but this solution worked for me