I had the same problem. When I experienced the problems my settings were:
- RAM 4GB
- CPU 3
- PAE/NX Enabled
- VT-x/AMD-V Enabled
- Nested Paging Enabled
- 3D Acceleration Disabled
- 2D Video Acceleration Disabled
I changed the following settings:
That made it work.
I changed the following setting:
It still worked.
Looked like there is 'something' with the number of processors. Based on the other answers, there does not seem to be a 'right' number that works for every system.
I then realized that I also have an Ubuntu 18.04 Server VM on my system. This one still works. I noticed that for this virtual machine the Paravirtualization Interface is set to Default, while for my Desktop VM (the one I was having problems with), it was set to Legacy.
I changed it to Default and now it works, with any number of CPU's.
So the solution, for me, was to:
Set Paravirtualization Interface to Default.