virtualbox.org

I got that message running my VM on which I have Ubuntu LTS 20.04

That log it mentions might be helpful, zipped, and attached as you posted the screenshot, please.

I attached it.

You get a triple fault the instant the guest tries to boot from the hard disk. Possibly implies disk corruption.

I also don't like the fact that you are using 3.3GB of the 3.4GB free RAM on your PC. Your PC only has 8GB to start with, so 3GB to a VM (plus 256MB graphics RAM) is always going to be a stretch - reduce to 2GB, and keep an eye on how many RAM hungry apps you have open on the host.

So, do you suggest me to reduce the used RAM to 2GB in order to try ro recover the VM ?

I reduced to 2GB, but I'm still not be able to start the VM

Try upgrading to the latest version of VirtualBox (6.1.16), remember to upgrade the extension pack too, and provide a new zipped log if the issue continues.

However, I'll point out that I didn't say that fixing the RAM issue would fix your problem - that was a separate issue that I picked up. The most likely cause of a triple fault error the instant a PC it tries to boot from hard disk is some kind of corruption of the boot code, or perhaps the OS was never installed correctly on the current host. I assume that you don't have a working backup?

I solved the issue reducing the used RAM and upgrading VirtualBox version and extension pack. Thank you!