I've been running Ubuntu 18.04 on VirtualBox for 5 months now, with no problems.
Today, I've encountered a quite annoying problem, to be honest. After I log in, the screen goes black with no reaction from mouse or keyboard input.
After a while though, the login screen reappears, but if I log in again the same thing happens, over and over again. I've already increased my Video memory in settings but that doesn't help.
Also, I wonder why this is happening now, although it never happened in the previous 5 months. Note that I only use this virtual machine for school stuff.