Comment 6 for bug 1772246

I tested an upgrade from a fresh and stock install of Lubuntu 16.04 to Lubuntu 18.04 today and the upgrade was successfully calcluated (while yours was not). I also tried installed vlc and some gnome packages you had installed and could not recreate the failure.

I suspect your failure had to do with the xorg packages you had installed from the oibaf PPA. PPA packages regularly cause issues when upgrading from one release to another.