During boot of your system and just after the BIOS messages Ubuntu might display a black screen with an error: no video mode activated message. The system boots normally but it is annoying.
GRUB by default hides the boot menu and this message can show up if there is no video mode set. It seems to be a known bug when the hidden option is active. I think you can just dismiss is it with ESC or ignore it as it has no adverse effect.
If you find it annoying you can get rid of it with the following steps.
Activate the menu option in GRUB.
Make the following changes. Adjust your timeout to the number of seconds you want the boot menu to be displayed. Values below 5 seconds are difficult to catch.
#GRUB_TIMEOUT_STYLE=hidden GRUB_TIMEOUT_STYLE=menu GRUB_TIMEOUT=10
Update the changes:
$ sudo update-grub
Reboot and check that the changes have been applied correctly.