I just updated my backup system (18.04.3) and my display (1920x1080) went into 800x600 mode and I am unable to correct. In the control panel the display shows as unknown and I can't change any settings. It has an Nvidia Quadro 600 graphics card.
I booted into Clear Linux and Ubuntu on other drives and everything was fine. I changed the Quadro out with a Radeon RX 460, booted into the original UM disk and everything was back to normal.
I installed Ubuntu Mate 18.04.3 onto a different drive. The installation program looked fine (I checked the box to install updates during the installation). When I rebooted to the fresh installation, I had the same 800x600 problem.
Is there something recent with UM that has changed to cause this problem with Nvidia? Is there something with the configuration I can try? All of my other machines with UM aren't experiencing this (none have Nvidia graphics). I keep my back up machine up to date, roughly weekly so this seems to be recent.