The solution

In the Ubuntu console window, open the following file in your editor of choice:

sudo vim /etc/default/grub

Uncomment the following line and set another resolution for it, for instance 1024×768:

GRUB_GFXMODE=1024×768

Next, write this new information for the next boot with the command:

sudo update-grub2

And then reboot to see the effect:

sudo shutdown -r 0